The Marine Life Information Network

Global warming (extreme)

Extreme emission scenario (by the end of this century 2081-2100) benchmark of:

• A 5°C rise in SST and NBT (coastal to the shelf seas),

• A 6°C rise in surface air temperature (in eulittoral and supralittoral habitats).

• A 1°C rise in Deep-sea habitats (>200 m) off the continental shelf, and

• A 5°C rise in surface air temperature in intertidal habitats exclusive to Scotland. Further detail.

Evidence

The distribution of kelp is strongly influenced by climatic conditions; therefore, kelp species are extremely sensitive to the ongoing ocean warming (Kain, 1979; Van Den Hoek, 1982; Breeman, 1990; Lüning, 1990; Assis et al., 2016; Smale, 2020). Northern distribution boundaries are set by winter temperatures that are lethal, or summer temperatures too low for growth and/or reproduction, while southern limits are set by high lethal summer temperatures or winter temperatures too high for induction of a crucial step in the life cycle (Breeman, 1990). Kelps have a high dependence on ocean temperatures, which makes them highly vulnerable to ocean warming (Assis et al., 2014). As temperatures increase, populations found towards the upper limit of their temperature range may be adversely affected by warming as physiological thresholds are exceeded (Wiens, 2016). Thermal stress can lead to mortality and consequent population-level effects, such as decreased abundance, altered size structure, local extinction and range contractions (Smale, 2020). 

Climate change is projected to increase the average sea surface temperature by between 1 and 3°C over the 21^st century and is predicted to cause the northward retreat of kelps (Solomon et al., 2007, Méléder et al., 2010 and Raybaud et al., 2013 cited in Kerrison et al., 2015).

Saccharina latissima is a polar to temperate macroalgae distributed from Greenland to the coast of Portugal, and in the north west Atlantic is found as far south as New York State, USA. In the UK, sea surface temperatures range between 6 and 19°C (Huthnance, 2010), and Saccharina latissima is in the middle of its biogeographic range. At its southern distribution in New York, temperatures can regularly reach ≥20°C for six weeks or more during summer months (Gerard & Du Bois, 1988). Saccharina latissima has already experienced some abundance and distribution changes due to a warming climate; mainly a decrease at the rear edges on both sides of the Atlantic and an increase in abundance at the polar regions (Diehl et al., 2023; Feehan et al., 2019 and Filbee-Dexter et al., 2016 cited in Veenhof et al., 2024). In Europe, Saccharina latissima has shifted poleward from Northern Europe (Moy & Christie, 2012; Simkanin et al., 2005 cited in Veenhof et al., 2024). Climate Velocity Trajectory (CVT) models by Veenhof et al. (2024) show further projected losses of seaweeds at warm edges of species ranges and gains at cold edges. Losses at warm edges were projected to be severe for some species, including the complete loss by 2070 of Saccharina latissima from northern Spain (Veenhof et al., 2024). Range expansions for Saccharina latissima may occur in the Russian Arctic, but less area appears suitable in Greenland and the Canadian Arctic (Veenhof et al., 2024). Therefore, more abundance and distribution shifts are increasingly expected in the future.

Yesson et al. (2015b) examined the change in abundance of large brown seaweeds in the British Isles between 1974 and 2010. They found that for all sites, Saccharina latissima and Chorda filum both showed a negative trend in abundance. Regional Sea Surface Temperatures showed annual fluctuations between 1974 and 2010, and the general trend has been a 1 to 2°C increase during this time-period, with the East coast (North Sea) experiencing the greatest increases (Yesson et al., 2015b). In addition, only the abundance of Saccharina latissima responded negatively to both summer and winter temperatures (Yesson et al., 2015b). Saccharina latissima has an optimal growth temperature between 10 and 15°C (Li et al., 2020), with growth reducing by 50 to 70% at 20°C, and all experimental specimens disintegrating after seven days at 23°C (Bolton & Lüning, 1982). The temperature isotherm of 19 to 20°C has been reported as limiting Saccharina latissima growth (Müller et al., 2009). Armitage et al. (2017) noted that Saccharina latissima was the most successful species in the cool summer (approx. 12 to 15°C), but it was strongly negatively affected by the hot summer (≥18°C), during a field study in southwestern Norway to observe competition between a non-native and two native habitat-building seaweeds.

Simonson, Scheibling & Metaxas (2015a) investigated the impacts of four temperature treatments (11, 14, 18 and 21°C) on growth, net length change and mortality of Saccharina latissima in Nova Scotia. Histological analysis showed temperature-mediated tissue damage, including holes, splitting of the medulla, damage to the meristoderm and loss of differentiation between tissue layers at temperatures between 14 and 21°C. Exposure to 21°C for one week reduced blade tissue strength (breaking stress) and extensibility (breaking strain) by 40 to 70% and exhibited reduced strength after three-week exposure to 18°C (Simonson, Scheibling & Metaxas, 2015a). Since the middle of the 20^th century, kelp species in Nova Scotia have experienced large population declines, up to 85 to 99%, of which temperature could have been a contributing factor (Filbee-Dexter, Feehan & Scheibling, 2016). However, Krumhansl et al. (2023) analysed the changes in Nova Scotia kelp abundance over the past 40 years (1982 to 2022) and found that there has been a loss in cold-tolerant kelps (such as Alaria esculenta, Saccorhiza dermatodea, and Agarum clathratum) and an increase in favour of the more warm-tolerant kelps like Saccharina latissima and Laminaria digitata. Kelp abundance increased since 2000, with Saccharina latissima widely abundant in the region by 2022 (Krumhansl et al., 2023). The highest kelp cover occurred on wave-exposed shores and at sites where temperatures have remained below thresholds for growth (21°C) and mortality (23 °C) (Krumhansl et al., 2023). Moreover, kelp has recovered from turf dominance following losses at some sites during a warm period from 2010 to 2012 (Krumhansl et al., 2023). Krumhansl et al. (2023) concluded that the dramatic change seen in kelp community composition in Nova Scotia over the past 40 years was in part driven by the loss of sea urchin herbivory, but a broad-scale shift to turf-dominance had not occurred, and that resilience and persistence were still a feature of kelp forests in the region despite rapid warming over the past several decades.

Elevated temperatures can increase erosion of Saccharina latissima blades and the subsequent release of total organic carbon and total nitrogen. Ding, Brussaard & Timmermans (2025) collected Saccharina latissima samples from the coastal waters south of Texel, The Netherlands, and subjected samples to naturally increased temperatures (from 16.1°C to 22.5°C) and further elevated temperatures (from 16.1°C to 27.1°C). A significant increase in the erosion rate of the distal parts of blades was observed in both temperature treatments, and substantial amounts (4.24 ± 0.31 mg/cm of carbon and 0.32 ± 0.13 mg/cm of nitrogen) of nutrients were released from Saccharina latissima, especially under sub-lethal temperature conditions. Under further elevated temperatures, with a prolonged period of higher temperature and a maximum temperature of 27.1°C, the effects were stronger, and erosion occurred along the edges of the whole blade. Ding, Brussaard & Timmermans (2025) concluded that rising temperatures accelerate the erosion of Saccharina latissima blades, highlighting a reason for the decline of kelp forests under climate change, as well as the potential impacts on nutrient cycling in the oceans.

Müller, Wiencke & Bischof (2008) found that elevated temperatures can exacerbate stress from ultraviolet radiation from sunlight. They investigated the combined effects of temperature and light quality on early life stages of Laminaria digitata and Saccharina latissima from Arctic (Spitsbergen) and temperate (Helgoland) populations. Temperature treatments ranged from 2°C to 18°C, representing Arctic summer conditions and North Sea summer extremes. For Laminaria digitata, Arctic populations germinated well at 2 to 12°C but failed at 18°C, while Helgoland populations showed optimal germination at 7 to 18°C. Saccharina latissima exhibited very low germination in Arctic populations (8 to 35%) and complete inhibition at 18°C, whereas temperate populations maintained high germination (85 to 92%) across all temperatures. UV-B radiation was the most damaging factor, reducing germination by up to 99% in Arctic Laminaria digitata and 74 to 90% in Arctic Saccharina latissima, and strongly inhibiting egg release (from 19 to 34 eggs mm² under normal light to 1.5 to 4 eggs mm² under UV-B). UV-A occasionally enhanced gametogenesis at moderate temperatures but did not offset UV-B damage. Overall, more light (UV exposure) combined with higher temperatures produced the greatest negative effects, while low light and moderate temperatures favoured Arctic populations, and these findings indicate that warming exacerbates UV-B stress and severely limits recruitment (Müller, Wiencke & Bischof, 2008).

In a warming experiment studying Arctic populations of Saccharina latissima, no gametophytes survived at 20°C, but most growth parameters were greater at 10 to 15°C than at 5°C (Park et al., 2017). Another warming experiment involving Saccharina latissima from Kongsfjorden (Svalbard, Norway) highlighted an increase in physiological performance and growth in samples at 15°C (compared to 0°C), and that at least Arctic populations of Saccharina latissima can adjust and might even benefit from increased temperatures (Li et al., 2020). However, Gordillo, Carmona & Jimenez (2022) observed how Arctic individuals of Saccharina latissima lost more biomass in the dark at higher temperatures than lower ones, with a warmer polar night posing a limit on multi-year seaweeds to occupy new ice-free illuminated areas of the Arctic coasts.

Temperature is an environmental factor controlling the development of the microscopic stages of Saccharina latissima, with crucial changes in survival, growth, and gametogenesis occurring within a few degrees of its upper thermal limits (Redmond, 2013). The optimal germination temperature for Saccharina latissima is between 2°C and 12°C, with gametophyte survival between 23 to 25°C (Müller et al., 2009). Germination rates drop at 22°C, with surviving gametophytes smaller than those grown at lower temperatures (Redmond, 2013). Park et al. (2017) observed reductions in the percentage of sporophytes produced at 15°C when compared to values produced at 5°C and 10°C. Fales et al. (2023) compared the physiological responses of Saccharina latissima sporophytes to high temperature stress (low: 9 and 13°C, moderate: 15 and 16°C, and warm: 21°C) and nitrogen limitation (low: 1 to 3 μM vs. high: >10 μM) over 8 to 9 days. Saccharina latissima responded negatively to elevated temperatures, but not to low nitrogen levels. Blades of Saccharina latissima showed signs of metabolic stress and reduced growth in the warmest temperature treatment (21°C), at both high and low nitrogen levels, suggesting that Saccharina latissima is susceptible to thermal stress over short time periods, and that nutrient additions may actually reduce kelp performance at supra-optimal temperatures (Fales et al., 2023).

Niedzwiedz et al. (2022) also studied the response of Saccharina latissima sporophytes (sampled from Helgoland, German Bight, in June 2018, August 2018 and August 2019) to warming (at treatment temperatures of 18, 20, 22 and 24°C) and found that survival decreased with increasing environmental and experimental temperatures. Growth also revealed seasonal patterns, being higher in June than in August (Niedzwiedz et al., 2022). Niedzwiedz et al. (2022) concluded that the thermal tolerance of Saccharina latissima towards heatwaves in summer is significantly affected by the environmental history it previously experienced. This result has been seen in other experiments involving Saccharina latissima as well, whereby its sporophytes are pre-exposed to moderate stress to improve the performance and tolerance of plants when exposed to harsher conditions. This is known as thermal priming, and this may happen naturally as kelp are continually exposed to a warming climate. Gauci et al. (2024) observed how gametophytes primed at 20°C for four and six weeks exhibited an 11-day longer tolerance at 22°C, a seven-day longer tolerance at 23°C, and a 1°C higher thermal tolerance over seven days compared to two-week priming.

In the field, Saccharina latissima has shown significant regional variation in its acclimation response to changing environmental conditions. For example, Gerard & Dubois (1988) observed sporophytes of Saccharina latissima that were regularly exposed to ≥20°C tolerated these high temperatures, whereas sporophytes from other populations, which rarely experience ≥17°C, showed 100% mortality after three weeks of exposure to 20°C. At higher temperatures (11, 18 and 21°C), the nutritional content (C/N) of Saccharina latissima seems unaffected (Simonson, Scheibling & Metaxas, 2015b). However, the sea snail Lacuna vincta was observed grazing more kelp at higher temperatures (21°C) and suggests that the effects of grazing will act additively with the direct effects of temperature and cause increased biomass loss from kelp beds (Simonson, Scheibling & Metaxas, 2015b).

Saccharina latissima has suffered a dramatic decline in the Skagerrak region, Norway, where community structure has shifted from Saccharina latissima forests to communities dominated by filamentous macroalgae (Moy & Christie, 2012). In 2006, Andersen et al. (2011) transplanted Saccharina latissima into areas from where this species had been lost previously to determine whether the kelp could grow and mature. High mortality occurred from August to November each year. In 2008, only six of the seventeen original transplanted Saccharina latissima sporophytes survived (approx. 65% mortality rate). All surviving sporophytes were heavily fouled by epiphytic organisms (estimated cover of 80 & 100%). Between 1960 and 2009, sea surface temperatures in the region had regularly exceeded 20°C, and so had the duration at which temperatures remained above 20°C. High sea temperatures have been linked to the slow growth of Saccharina latissima, which is likely due to a decrease in the photosynthetic ability of Saccharina latissima, and an increase in vulnerability to epiphytic loading, bacterial and viral attacks (Anderson et al., 2011).

Kelp forests, including populations of Saccharina latissima, across the coastline of New England, USA, have experienced population shifts since the start of the 21^st century. Suskiewicz et al. (2024) surveyed between 31 and 67 forests spanning >350 km of coastline in Maine between 2001 and 2018 and then modelled how temperature change and sea urchin density influenced kelp abundance. Notably, the time-period studied was marked by rapid regional warming and several marine heatwaves, and the length of coastline examined experiences a more than 6°C difference in summer seawater temperatures from north to south (Suskiewicz et al., 2024). The maximum summer Near-Surface Seawater Temperatures in southern Maine commonly exceeded 20°C and were, on average, approx. 5.6°C warmer than those observed in northeast Maine (Suskiewicz et al., 2024). Consequently, southwestern subregions now regularly experience temperatures (15°C) at which nitrate saturation reaches zero (García-Reyes et al., 2022 and Zimmerman & Kremer, 1984 cited in Suskiewicz et al., 2024) as well as temperatures (20°C) at which sugar kelp erodes faster than it grows (Lee & Brinkhuis, 1986, cited in Suskiewicz et al., 2024). Also, high seawater temperatures reduce nutrient availability to kelp, causing nutrient depletion at 15°C (García-Reyes et al., 2022 and Zimmerman & Kremer, 1984 cited in Suskiewicz et al., 2024); and reduced nutrients during periods of maximum growth (spring) or thermal stress (summer) can accelerate kelp loss over time, as seen across all subregions by the end of the study by Suskiewicz et al. (2024). Although forests (Saccharina latissima and Laminaria digitata) had broadly returned to Maine in the late 20^th century, forests in northeast Maine have since experienced slow but significant declines in kelp, and forest persistence in the northeast was juxtaposed by a rapid, widespread collapse in the southwest (Suskiewicz et al., 2024). Forests collapsed in the southwest likely because ocean warming has directly and indirectly made this area inhospitable to kelp (Suskiewicz et al., 2024).

Hill et al. (2025) used species distribution models to evaluate the potential of enhanced thermal tolerance to buffer the effects of climate change (an increase of 1 to 5°C in maximum sea surface temperature) on cold-adapted kelp species. The models demonstrated that an increase of 1 to 2°C in thermal tolerance could recover over 50% of predicted losses of suitable habitat for cold-adapted kelps, with Saccharina latissima peaking at 17°C (Hill et al., 2025). For example, in the East Atlantic, Saccharina latissima recovery was concentrated in the southeast UK, but all species had projected patches of recovery on the Iberian coastline (Hill et al., 2025). In the North Sea and Skagerrak regions, a tolerance increase of 4 to 5°C was required for complete recovery (Hill et al., 2025). In the Baltic Sea, Saccharina latissima recovered with a tolerance increase of 1 to 2°C except for the mouth of the Baltic, where some areas remained unrecovered, even with a 5°C increase in tolerance (Hill et al., 2025). Overall, Saccharina latissima had the highest recovery potential with 99% of its projected lost suitable habitat area recovered under all climate change scenarios explored using the species distribution models (Hill et al., 2025). However, relying on mitigation or adaptation alone will likely be insufficient to maintain their historic range under projected climate change (Hill et al., 2025).

Similarly, Goldsmit et al. (2021) used a Random Forest model to predict future habitat suitability and cover for the dominant kelp species under climate change scenarios in the Eastern Canadian Arctic. Saccharina latissima is projected to have the largest gain in suitable habitat in both 2050 and 2100, with declines projected for some areas (e.g., north of Baffin Bay, Foxe Basin and Hudson Bay) by 2100 (Goldsmit et al., 2021). In general, suitable habitat is projected to occur in the northernmost reaches of the Eastern Canadian Arctic and is expected to persist into the future (Goldsmit et al., 2021). As the ocean warms and ice recedes, the model by Goldsmit et al. (2021) projects that Saccharina latissima will gain suitable habitat along much of the west coast of Greenland and the northern arm of the Northwest Passage.

Assis et al. (2018) predicted that, under the highest emission scenario (RCP 8.5), the range of Saccharina latissima would move northwards, retreating from the coast of Portugal, France and the southwest coast of the UK. The authors projected that, under RCP 2.6, 13% suitable Laminaria hyperborea habitat would be lost from the Western English Channel, while under the RCP 8.5 emission, 87% of suitable habitat was expected to be lost.

Chorda filum is a cold boreal species, with a wide geographical distribution along the Arctic, Atlantic and Pacific coasts (www.obis.org). Chorda filum has been reported to have relatively good growth between the temperatures of 5°C and 15°C but reduced or inhibited growth at 20°C (Kawai et al., 2000). Chorda filum has an upper temperature tolerance of 26-28°C (Dieck, 1993). Although Lüning (1980) observed that between the temperatures of 15 and 20°C, Chorda filum could not reproduce, but found that sporophytes could tolerate ≤26°C. In addition, Lüning (1990) reported gametogenesis to occur at temperatures between 5°C and 10°C in the autumn months. 

Wilson et al. (2015) reported that an increase in sea surface temperature from 1974 to 2010 resulted in biogeographical changes, with declines in abundance of Chorda filum, particularly in the English Channel. Wilson et al. (2015) suggested the declines of Chorda filum could be because the summer temperatures in those southern regions were too high for gametogenesis. 

Many of the red algae species associated with the understorey turf can tolerate warm water temperatures. 

Sensitivity Assessment. UK populations of Saccharina latissima are found in the middle of the species distribution and are known to be able to survive at higher temperatures than currently experienced around the UK. The ability to tolerate summer seawater temperatures of >20°C in populations at their southern geographic limit is thought to be a genetic adaptation (Gerard & Du Bois, 1988) and may be crucial in the persistence of this species around the UK, as seawater temperatures rise.

With sea surface temperature around the UK of between 6 and 19°C (Huthnance, 2010), populations of Saccharina latissima and the understorey community of mixed red seaweeds may be able to adapt to cope with a gradual rise in ocean temperatures of 3°C (middle emission scenario) by the end of this century, leading to maximum summer high temperatures in the south of the UK of 22°C. However, increasing temperatures are likely to lead to a decrease in growth and some mortality. Therefore, resistance is assessed as ‘Medium’, and resilience is assessed as ‘Very Low’, as the loss is likely to be a long-term decline, due to the long-term nature of ocean warming. Therefore, this biotope is assessed as ‘Medium’ sensitivity to ocean warming in the middle emission scenario.

For the high emission scenario and extreme scenario, whereby sea temperatures rise by 4-5°C to potential southern summer temperatures of 23-24°C by the end of this century, Saccharina latissima is likely to be lost from southern England, as gametophytes are not thought to be able to survive at temperatures ≥23°C. This assessment corresponds with the results of ecological niche modelling by Assis et al. (2018), who predicted that Saccharina latissima would be lost from the southwest coast of the UK because of climate change. Therefore, resistance is assessed as ‘Low’, and resilience is assessed as ‘Very low’. This biotope is assessed as having ‘High’ sensitivity to ocean warming in the high and extreme emission scenarios.

Global warming (high)

High emission scenario (by the end of this century 2081-2100) benchmark of:

• A 4°C rise in SST, NBT (coastal to the shelf seas) and surface air temperature (in eulittoral and supralittoral habitats).

• A 1°C rise in Deep-sea habitats (>200 m) off the continental shelf, and

• A 3°C rise in surface air temperature in intertidal habitats exclusive to Scotland. Further detail.

Evidence

The distribution of kelp is strongly influenced by climatic conditions; therefore, kelp species are extremely sensitive to the ongoing ocean warming (Kain, 1979; Van Den Hoek, 1982; Breeman, 1990; Lüning, 1990; Assis et al., 2016; Smale, 2020). Northern distribution boundaries are set by winter temperatures that are lethal, or summer temperatures too low for growth and/or reproduction, while southern limits are set by high lethal summer temperatures or winter temperatures too high for induction of a crucial step in the life cycle (Breeman, 1990). Kelps have a high dependence on ocean temperatures, which makes them highly vulnerable to ocean warming (Assis et al., 2014). As temperatures increase, populations found towards the upper limit of their temperature range may be adversely affected by warming as physiological thresholds are exceeded (Wiens, 2016). Thermal stress can lead to mortality and consequent population-level effects, such as decreased abundance, altered size structure, local extinction and range contractions (Smale, 2020). 

Climate change is projected to increase the average sea surface temperature by between 1 and 3°C over the 21^st century and is predicted to cause the northward retreat of kelps (Solomon et al., 2007, Méléder et al., 2010 and Raybaud et al., 2013 cited in Kerrison et al., 2015).

Saccharina latissima is a polar to temperate macroalgae distributed from Greenland to the coast of Portugal, and in the north west Atlantic is found as far south as New York State, USA. In the UK, sea surface temperatures range between 6 and 19°C (Huthnance, 2010), and Saccharina latissima is in the middle of its biogeographic range. At its southern distribution in New York, temperatures can regularly reach ≥20°C for six weeks or more during summer months (Gerard & Du Bois, 1988). Saccharina latissima has already experienced some abundance and distribution changes due to a warming climate; mainly a decrease at the rear edges on both sides of the Atlantic and an increase in abundance at the polar regions (Diehl et al., 2023; Feehan et al., 2019 and Filbee-Dexter et al., 2016 cited in Veenhof et al., 2024). In Europe, Saccharina latissima has shifted poleward from Northern Europe (Moy & Christie, 2012; Simkanin et al., 2005 cited in Veenhof et al., 2024). Climate Velocity Trajectory (CVT) models by Veenhof et al. (2024) show further projected losses of seaweeds at warm edges of species ranges and gains at cold edges. Losses at warm edges were projected to be severe for some species, including the complete loss by 2070 of Saccharina latissima from northern Spain (Veenhof et al., 2024). Range expansions for Saccharina latissima may occur in the Russian Arctic, but less area appears suitable in Greenland and the Canadian Arctic (Veenhof et al., 2024). Therefore, more abundance and distribution shifts are increasingly expected in the future.

Yesson et al. (2015b) examined the change in abundance of large brown seaweeds in the British Isles between 1974 and 2010. They found that for all sites, Saccharina latissima and Chorda filum both showed a negative trend in abundance. Regional Sea Surface Temperatures showed annual fluctuations between 1974 and 2010, and the general trend has been a 1 to 2°C increase during this time-period, with the East coast (North Sea) experiencing the greatest increases (Yesson et al., 2015b). In addition, only the abundance of Saccharina latissima responded negatively to both summer and winter temperatures (Yesson et al., 2015b). Saccharina latissima has an optimal growth temperature between 10 and 15°C (Li et al., 2020), with growth reducing by 50 to 70% at 20°C, and all experimental specimens disintegrating after seven days at 23°C (Bolton & Lüning, 1982). The temperature isotherm of 19 to 20°C has been reported as limiting Saccharina latissima growth (Müller et al., 2009). Armitage et al. (2017) noted that Saccharina latissima was the most successful species in the cool summer (approx. 12 to 15°C), but it was strongly negatively affected by the hot summer (≥18°C), during a field study in southwestern Norway to observe competition between a non-native and two native habitat-building seaweeds.

Simonson, Scheibling & Metaxas (2015a) investigated the impacts of four temperature treatments (11, 14, 18 and 21°C) on growth, net length change and mortality of Saccharina latissima in Nova Scotia. Histological analysis showed temperature-mediated tissue damage, including holes, splitting of the medulla, damage to the meristoderm and loss of differentiation between tissue layers at temperatures between 14 and 21°C. Exposure to 21°C for one week reduced blade tissue strength (breaking stress) and extensibility (breaking strain) by 40 to 70% and exhibited reduced strength after three-week exposure to 18°C (Simonson, Scheibling & Metaxas, 2015a). Since the middle of the 20^th century, kelp species in Nova Scotia have experienced large population declines, up to 85 to 99%, of which temperature could have been a contributing factor (Filbee-Dexter, Feehan & Scheibling, 2016). However, Krumhansl et al. (2023) analysed the changes in Nova Scotia kelp abundance over the past 40 years (1982 to 2022) and found that there has been a loss in cold-tolerant kelps (such as Alaria esculenta, Saccorhiza dermatodea, and Agarum clathratum) and an increase in favour of the more warm-tolerant kelps like Saccharina latissima and Laminaria digitata. Kelp abundance increased since 2000, with Saccharina latissima widely abundant in the region by 2022 (Krumhansl et al., 2023). The highest kelp cover occurred on wave-exposed shores and at sites where temperatures have remained below thresholds for growth (21°C) and mortality (23 °C) (Krumhansl et al., 2023). Moreover, kelp has recovered from turf dominance following losses at some sites during a warm period from 2010 to 2012 (Krumhansl et al., 2023). Krumhansl et al. (2023) concluded that the dramatic change seen in kelp community composition in Nova Scotia over the past 40 years was in part driven by the loss of sea urchin herbivory, but a broad-scale shift to turf-dominance had not occurred, and that resilience and persistence were still a feature of kelp forests in the region despite rapid warming over the past several decades.

Elevated temperatures can increase erosion of Saccharina latissima blades and the subsequent release of total organic carbon and total nitrogen. Ding, Brussaard & Timmermans (2025) collected Saccharina latissima samples from the coastal waters south of Texel, The Netherlands, and subjected samples to naturally increased temperatures (from 16.1°C to 22.5°C) and further elevated temperatures (from 16.1°C to 27.1°C). A significant increase in the erosion rate of the distal parts of blades was observed in both temperature treatments, and substantial amounts (4.24 ± 0.31 mg/cm of carbon and 0.32 ± 0.13 mg/cm of nitrogen) of nutrients were released from Saccharina latissima, especially under sub-lethal temperature conditions. Under further elevated temperatures, with a prolonged period of higher temperature and a maximum temperature of 27.1°C, the effects were stronger, and erosion occurred along the edges of the whole blade. Ding, Brussaard & Timmermans (2025) concluded that rising temperatures accelerate the erosion of Saccharina latissima blades, highlighting a reason for the decline of kelp forests under climate change, as well as the potential impacts on nutrient cycling in the oceans.

Müller, Wiencke & Bischof (2008) found that elevated temperatures can exacerbate stress from ultraviolet radiation from sunlight. They investigated the combined effects of temperature and light quality on early life stages of Laminaria digitata and Saccharina latissima from Arctic (Spitsbergen) and temperate (Helgoland) populations. Temperature treatments ranged from 2°C to 18°C, representing Arctic summer conditions and North Sea summer extremes. For Laminaria digitata, Arctic populations germinated well at 2 to 12°C but failed at 18°C, while Helgoland populations showed optimal germination at 7 to 18°C. Saccharina latissima exhibited very low germination in Arctic populations (8 to 35%) and complete inhibition at 18°C, whereas temperate populations maintained high germination (85 to 92%) across all temperatures. UV-B radiation was the most damaging factor, reducing germination by up to 99% in Arctic Laminaria digitata and 74 to 90% in Arctic Saccharina latissima, and strongly inhibiting egg release (from 19 to 34 eggs mm² under normal light to 1.5 to 4 eggs mm² under UV-B). UV-A occasionally enhanced gametogenesis at moderate temperatures but did not offset UV-B damage. Overall, more light (UV exposure) combined with higher temperatures produced the greatest negative effects, while low light and moderate temperatures favoured Arctic populations, and these findings indicate that warming exacerbates UV-B stress and severely limits recruitment (Müller, Wiencke & Bischof, 2008).

In a warming experiment studying Arctic populations of Saccharina latissima, no gametophytes survived at 20°C, but most growth parameters were greater at 10 to 15°C than at 5°C (Park et al., 2017). Another warming experiment involving Saccharina latissima from Kongsfjorden (Svalbard, Norway) highlighted an increase in physiological performance and growth in samples at 15°C (compared to 0°C), and that at least Arctic populations of Saccharina latissima can adjust and might even benefit from increased temperatures (Li et al., 2020). However, Gordillo, Carmona & Jimenez (2022) observed how Arctic individuals of Saccharina latissima lost more biomass in the dark at higher temperatures than lower ones, with a warmer polar night posing a limit on multi-year seaweeds to occupy new ice-free illuminated areas of the Arctic coasts.

Temperature is an environmental factor controlling the development of the microscopic stages of Saccharina latissima, with crucial changes in survival, growth, and gametogenesis occurring within a few degrees of its upper thermal limits (Redmond, 2013). The optimal germination temperature for Saccharina latissima is between 2°C and 12°C, with gametophyte survival between 23 to 25°C (Müller et al., 2009). Germination rates drop at 22°C, with surviving gametophytes smaller than those grown at lower temperatures (Redmond, 2013). Park et al. (2017) observed reductions in the percentage of sporophytes produced at 15°C when compared to values produced at 5°C and 10°C. Fales et al. (2023) compared the physiological responses of Saccharina latissima sporophytes to high temperature stress (low: 9 and 13°C, moderate: 15 and 16°C, and warm: 21°C) and nitrogen limitation (low: 1 to 3 μM vs. high: >10 μM) over 8 to 9 days. Saccharina latissima responded negatively to elevated temperatures, but not to low nitrogen levels. Blades of Saccharina latissima showed signs of metabolic stress and reduced growth in the warmest temperature treatment (21°C), at both high and low nitrogen levels, suggesting that Saccharina latissima is susceptible to thermal stress over short time periods, and that nutrient additions may actually reduce kelp performance at supra-optimal temperatures (Fales et al., 2023).

Niedzwiedz et al. (2022) also studied the response of Saccharina latissima sporophytes (sampled from Helgoland, German Bight, in June 2018, August 2018 and August 2019) to warming (at treatment temperatures of 18, 20, 22 and 24°C) and found that survival decreased with increasing environmental and experimental temperatures. Growth also revealed seasonal patterns, being higher in June than in August (Niedzwiedz et al., 2022). Niedzwiedz et al. (2022) concluded that the thermal tolerance of Saccharina latissima towards heatwaves in summer is significantly affected by the environmental history it previously experienced. This result has been seen in other experiments involving Saccharina latissima as well, whereby its sporophytes are pre-exposed to moderate stress to improve the performance and tolerance of plants when exposed to harsher conditions. This is known as thermal priming, and this may happen naturally as kelp are continually exposed to a warming climate. Gauci et al. (2024) observed how gametophytes primed at 20°C for four and six weeks exhibited an 11-day longer tolerance at 22°C, a seven-day longer tolerance at 23°C, and a 1°C higher thermal tolerance over seven days compared to two-week priming.

In the field, Saccharina latissima has shown significant regional variation in its acclimation response to changing environmental conditions. For example, Gerard & Dubois (1988) observed sporophytes of Saccharina latissima that were regularly exposed to ≥20°C tolerated these high temperatures, whereas sporophytes from other populations, which rarely experience ≥17°C, showed 100% mortality after three weeks of exposure to 20°C. At higher temperatures (11, 18 and 21°C), the nutritional content (C/N) of Saccharina latissima seems unaffected (Simonson, Scheibling & Metaxas, 2015b). However, the sea snail Lacuna vincta was observed grazing more kelp at higher temperatures (21°C) and suggests that the effects of grazing will act additively with the direct effects of temperature and cause increased biomass loss from kelp beds (Simonson, Scheibling & Metaxas, 2015b).

Saccharina latissima has suffered a dramatic decline in the Skagerrak region, Norway, where community structure has shifted from Saccharina latissima forests to communities dominated by filamentous macroalgae (Moy & Christie, 2012). In 2006, Andersen et al. (2011) transplanted Saccharina latissima into areas from where this species had been lost previously to determine whether the kelp could grow and mature. High mortality occurred from August to November each year. In 2008, only six of the seventeen original transplanted Saccharina latissima sporophytes survived (approx. 65% mortality rate). All surviving sporophytes were heavily fouled by epiphytic organisms (estimated cover of 80 & 100%). Between 1960 and 2009, sea surface temperatures in the region had regularly exceeded 20°C, and so had the duration at which temperatures remained above 20°C. High sea temperatures have been linked to the slow growth of Saccharina latissima, which is likely due to a decrease in the photosynthetic ability of Saccharina latissima, and an increase in vulnerability to epiphytic loading, bacterial and viral attacks (Anderson et al., 2011).

Kelp forests, including populations of Saccharina latissima, across the coastline of New England, USA, have experienced population shifts since the start of the 21^st century. Suskiewicz et al. (2024) surveyed between 31 and 67 forests spanning >350 km of coastline in Maine between 2001 and 2018 and then modelled how temperature change and sea urchin density influenced kelp abundance. Notably, the time-period studied was marked by rapid regional warming and several marine heatwaves, and the length of coastline examined experiences a more than 6°C difference in summer seawater temperatures from north to south (Suskiewicz et al., 2024). The maximum summer Near-Surface Seawater Temperatures in southern Maine commonly exceeded 20°C and were, on average, approx. 5.6°C warmer than those observed in northeast Maine (Suskiewicz et al., 2024). Consequently, southwestern subregions now regularly experience temperatures (15°C) at which nitrate saturation reaches zero (García-Reyes et al., 2022 and Zimmerman & Kremer, 1984 cited in Suskiewicz et al., 2024) as well as temperatures (20°C) at which sugar kelp erodes faster than it grows (Lee & Brinkhuis, 1986, cited in Suskiewicz et al., 2024). Also, high seawater temperatures reduce nutrient availability to kelp, causing nutrient depletion at 15°C (García-Reyes et al., 2022 and Zimmerman & Kremer, 1984 cited in Suskiewicz et al., 2024); and reduced nutrients during periods of maximum growth (spring) or thermal stress (summer) can accelerate kelp loss over time, as seen across all subregions by the end of the study by Suskiewicz et al. (2024). Although forests (Saccharina latissima and Laminaria digitata) had broadly returned to Maine in the late 20^th century, forests in northeast Maine have since experienced slow but significant declines in kelp, and forest persistence in the northeast was juxtaposed by a rapid, widespread collapse in the southwest (Suskiewicz et al., 2024). Forests collapsed in the southwest likely because ocean warming has directly and indirectly made this area inhospitable to kelp (Suskiewicz et al., 2024).

Hill et al. (2025) used species distribution models to evaluate the potential of enhanced thermal tolerance to buffer the effects of climate change (an increase of 1 to 5°C in maximum sea surface temperature) on cold-adapted kelp species. The models demonstrated that an increase of 1 to 2°C in thermal tolerance could recover over 50% of predicted losses of suitable habitat for cold-adapted kelps, with Saccharina latissima peaking at 17°C (Hill et al., 2025). For example, in the East Atlantic, Saccharina latissima recovery was concentrated in the southeast UK, but all species had projected patches of recovery on the Iberian coastline (Hill et al., 2025). In the North Sea and Skagerrak regions, a tolerance increase of 4 to 5°C was required for complete recovery (Hill et al., 2025). In the Baltic Sea, Saccharina latissima recovered with a tolerance increase of 1 to 2°C except for the mouth of the Baltic, where some areas remained unrecovered, even with a 5°C increase in tolerance (Hill et al., 2025). Overall, Saccharina latissima had the highest recovery potential with 99% of its projected lost suitable habitat area recovered under all climate change scenarios explored using the species distribution models (Hill et al., 2025). However, relying on mitigation or adaptation alone will likely be insufficient to maintain their historic range under projected climate change (Hill et al., 2025).

Similarly, Goldsmit et al. (2021) used a Random Forest model to predict future habitat suitability and cover for the dominant kelp species under climate change scenarios in the Eastern Canadian Arctic. Saccharina latissima is projected to have the largest gain in suitable habitat in both 2050 and 2100, with declines projected for some areas (e.g., north of Baffin Bay, Foxe Basin and Hudson Bay) by 2100 (Goldsmit et al., 2021). In general, suitable habitat is projected to occur in the northernmost reaches of the Eastern Canadian Arctic and is expected to persist into the future (Goldsmit et al., 2021). As the ocean warms and ice recedes, the model by Goldsmit et al. (2021) projects that Saccharina latissima will gain suitable habitat along much of the west coast of Greenland and the northern arm of the Northwest Passage.

Assis et al. (2018) predicted that, under the highest emission scenario (RCP 8.5), the range of Saccharina latissima would move northwards, retreating from the coast of Portugal, France and the southwest coast of the UK. The authors projected that, under RCP 2.6, 13% suitable Laminaria hyperborea habitat would be lost from the Western English Channel, while under the RCP 8.5 emission, 87% of suitable habitat was expected to be lost.

Chorda filum is a cold boreal species, with a wide geographical distribution along the Arctic, Atlantic and Pacific coasts (www.obis.org). Chorda filum has been reported to have relatively good growth between the temperatures of 5°C and 15°C but reduced or inhibited growth at 20°C (Kawai et al., 2000). Chorda filum has an upper temperature tolerance of 26-28°C (Dieck, 1993). Although Lüning (1980) observed that between the temperatures of 15 and 20°C, Chorda filum could not reproduce, but found that sporophytes could tolerate ≤26°C. In addition, Lüning (1990) reported gametogenesis to occur at temperatures between 5°C and 10°C in the autumn months. 

Wilson et al. (2015) reported that an increase in sea surface temperature from 1974 to 2010 resulted in biogeographical changes, with declines in abundance of Chorda filum, particularly in the English Channel. Wilson et al. (2015) suggested the declines of Chorda filum could be because the summer temperatures in those southern regions were too high for gametogenesis. 

Many of the red algae species associated with the understorey turf can tolerate warm water temperatures. 

Sensitivity Assessment. UK populations of Saccharina latissima are found in the middle of the species distribution and are known to be able to survive at higher temperatures than currently experienced around the UK. The ability to tolerate summer seawater temperatures of >20°C in populations at their southern geographic limit is thought to be a genetic adaptation (Gerard & Du Bois, 1988) and may be crucial in the persistence of this species around the UK, as seawater temperatures rise.

With sea surface temperature around the UK of between 6 and 19°C (Huthnance, 2010), populations of Saccharina latissima and the understorey community of mixed red seaweeds may be able to adapt to cope with a gradual rise in ocean temperatures of 3°C (middle emission scenario) by the end of this century, leading to maximum summer high temperatures in the south of the UK of 22°C. However, increasing temperatures are likely to lead to a decrease in growth and some mortality. Therefore, resistance is assessed as ‘Medium’, and resilience is assessed as ‘Very Low’, as the loss is likely to be a long-term decline, due to the long-term nature of ocean warming. Therefore, this biotope is assessed as ‘Medium’ sensitivity to ocean warming in the middle emission scenario.

For the high emission scenario and extreme scenario, whereby sea temperatures rise by 4-5°C to potential southern summer temperatures of 23-24°C by the end of this century, Saccharina latissima is likely to be lost from southern England, as gametophytes are not thought to be able to survive at temperatures ≥23°C. This assessment corresponds with the results of ecological niche modelling by Assis et al. (2018), who predicted that Saccharina latissima would be lost from the southwest coast of the UK because of climate change. Therefore, resistance is assessed as ‘Low’, and resilience is assessed as ‘Very low’. This biotope is assessed as having ‘High’ sensitivity to ocean warming in the high and extreme emission scenarios.

Global warming (middle)

Middle emission scenario (by the end of this century 2081-2100) benchmark of:

• A 3°C rise in SST, NBT (coastal to the shelf seas) and surface air temperature (in eulittoral and supralittoral habitats).

• A 1°C rise in Deep-sea habitats (>200 m) off the continental shelf.

• A 2°C rise in surface air temperature in intertidal habitats exclusive to Scotland. Further detail.

Evidence

The distribution of kelp is strongly influenced by climatic conditions; therefore, kelp species are extremely sensitive to the ongoing ocean warming (Kain, 1979; Van Den Hoek, 1982; Breeman, 1990; Lüning, 1990; Assis et al., 2016; Smale, 2020). Northern distribution boundaries are set by winter temperatures that are lethal, or summer temperatures too low for growth and/or reproduction, while southern limits are set by high lethal summer temperatures or winter temperatures too high for induction of a crucial step in the life cycle (Breeman, 1990). Kelps have a high dependence on ocean temperatures, which makes them highly vulnerable to ocean warming (Assis et al., 2014). As temperatures increase, populations found towards the upper limit of their temperature range may be adversely affected by warming as physiological thresholds are exceeded (Wiens, 2016). Thermal stress can lead to mortality and consequent population-level effects, such as decreased abundance, altered size structure, local extinction and range contractions (Smale, 2020). 

Climate change is projected to increase the average sea surface temperature by between 1 and 3°C over the 21^st century and is predicted to cause the northward retreat of kelps (Solomon et al., 2007, Méléder et al., 2010 and Raybaud et al., 2013 cited in Kerrison et al., 2015).

Saccharina latissima is a polar to temperate macroalgae distributed from Greenland to the coast of Portugal, and in the north west Atlantic is found as far south as New York State, USA. In the UK, sea surface temperatures range between 6 and 19°C (Huthnance, 2010), and Saccharina latissima is in the middle of its biogeographic range. At its southern distribution in New York, temperatures can regularly reach ≥20°C for six weeks or more during summer months (Gerard & Du Bois, 1988). Saccharina latissima has already experienced some abundance and distribution changes due to a warming climate; mainly a decrease at the rear edges on both sides of the Atlantic and an increase in abundance at the polar regions (Diehl et al., 2023; Feehan et al., 2019 and Filbee-Dexter et al., 2016 cited in Veenhof et al., 2024). In Europe, Saccharina latissima has shifted poleward from Northern Europe (Moy & Christie, 2012; Simkanin et al., 2005 cited in Veenhof et al., 2024). Climate Velocity Trajectory (CVT) models by Veenhof et al. (2024) show further projected losses of seaweeds at warm edges of species ranges and gains at cold edges. Losses at warm edges were projected to be severe for some species, including the complete loss by 2070 of Saccharina latissima from northern Spain (Veenhof et al., 2024). Range expansions for Saccharina latissima may occur in the Russian Arctic, but less area appears suitable in Greenland and the Canadian Arctic (Veenhof et al., 2024). Therefore, more abundance and distribution shifts are increasingly expected in the future.

Yesson et al. (2015b) examined the change in abundance of large brown seaweeds in the British Isles between 1974 and 2010. They found that for all sites, Saccharina latissima and Chorda filum both showed a negative trend in abundance. Regional Sea Surface Temperatures showed annual fluctuations between 1974 and 2010, and the general trend has been a 1 to 2°C increase during this time-period, with the East coast (North Sea) experiencing the greatest increases (Yesson et al., 2015b). In addition, only the abundance of Saccharina latissima responded negatively to both summer and winter temperatures (Yesson et al., 2015b). Saccharina latissima has an optimal growth temperature between 10 and 15°C (Li et al., 2020), with growth reducing by 50 to 70% at 20°C, and all experimental specimens disintegrating after seven days at 23°C (Bolton & Lüning, 1982). The temperature isotherm of 19 to 20°C has been reported as limiting Saccharina latissima growth (Müller et al., 2009). Armitage et al. (2017) noted that Saccharina latissima was the most successful species in the cool summer (approx. 12 to 15°C), but it was strongly negatively affected by the hot summer (≥18°C), during a field study in southwestern Norway to observe competition between a non-native and two native habitat-building seaweeds.

Simonson, Scheibling & Metaxas (2015a) investigated the impacts of four temperature treatments (11, 14, 18 and 21°C) on growth, net length change and mortality of Saccharina latissima in Nova Scotia. Histological analysis showed temperature-mediated tissue damage, including holes, splitting of the medulla, damage to the meristoderm and loss of differentiation between tissue layers at temperatures between 14 and 21°C. Exposure to 21°C for one week reduced blade tissue strength (breaking stress) and extensibility (breaking strain) by 40 to 70% and exhibited reduced strength after three-week exposure to 18°C (Simonson, Scheibling & Metaxas, 2015a). Since the middle of the 20^th century, kelp species in Nova Scotia have experienced large population declines, up to 85 to 99%, of which temperature could have been a contributing factor (Filbee-Dexter, Feehan & Scheibling, 2016). However, Krumhansl et al. (2023) analysed the changes in Nova Scotia kelp abundance over the past 40 years (1982 to 2022) and found that there has been a loss in cold-tolerant kelps (such as Alaria esculenta, Saccorhiza dermatodea, and Agarum clathratum) and an increase in favour of the more warm-tolerant kelps like Saccharina latissima and Laminaria digitata. Kelp abundance increased since 2000, with Saccharina latissima widely abundant in the region by 2022 (Krumhansl et al., 2023). The highest kelp cover occurred on wave-exposed shores and at sites where temperatures have remained below thresholds for growth (21°C) and mortality (23 °C) (Krumhansl et al., 2023). Moreover, kelp has recovered from turf dominance following losses at some sites during a warm period from 2010 to 2012 (Krumhansl et al., 2023). Krumhansl et al. (2023) concluded that the dramatic change seen in kelp community composition in Nova Scotia over the past 40 years was in part driven by the loss of sea urchin herbivory, but a broad-scale shift to turf-dominance had not occurred, and that resilience and persistence were still a feature of kelp forests in the region despite rapid warming over the past several decades.

Elevated temperatures can increase erosion of Saccharina latissima blades and the subsequent release of total organic carbon and total nitrogen. Ding, Brussaard & Timmermans (2025) collected Saccharina latissima samples from the coastal waters south of Texel, The Netherlands, and subjected samples to naturally increased temperatures (from 16.1°C to 22.5°C) and further elevated temperatures (from 16.1°C to 27.1°C). A significant increase in the erosion rate of the distal parts of blades was observed in both temperature treatments, and substantial amounts (4.24 ± 0.31 mg/cm of carbon and 0.32 ± 0.13 mg/cm of nitrogen) of nutrients were released from Saccharina latissima, especially under sub-lethal temperature conditions. Under further elevated temperatures, with a prolonged period of higher temperature and a maximum temperature of 27.1°C, the effects were stronger, and erosion occurred along the edges of the whole blade. Ding, Brussaard & Timmermans (2025) concluded that rising temperatures accelerate the erosion of Saccharina latissima blades, highlighting a reason for the decline of kelp forests under climate change, as well as the potential impacts on nutrient cycling in the oceans.

Müller, Wiencke & Bischof (2008) found that elevated temperatures can exacerbate stress from ultraviolet radiation from sunlight. They investigated the combined effects of temperature and light quality on early life stages of Laminaria digitata and Saccharina latissima from Arctic (Spitsbergen) and temperate (Helgoland) populations. Temperature treatments ranged from 2°C to 18°C, representing Arctic summer conditions and North Sea summer extremes. For Laminaria digitata, Arctic populations germinated well at 2 to 12°C but failed at 18°C, while Helgoland populations showed optimal germination at 7 to 18°C. Saccharina latissima exhibited very low germination in Arctic populations (8 to 35%) and complete inhibition at 18°C, whereas temperate populations maintained high germination (85 to 92%) across all temperatures. UV-B radiation was the most damaging factor, reducing germination by up to 99% in Arctic Laminaria digitata and 74 to 90% in Arctic Saccharina latissima, and strongly inhibiting egg release (from 19 to 34 eggs mm² under normal light to 1.5 to 4 eggs mm² under UV-B). UV-A occasionally enhanced gametogenesis at moderate temperatures but did not offset UV-B damage. Overall, more light (UV exposure) combined with higher temperatures produced the greatest negative effects, while low light and moderate temperatures favoured Arctic populations, and these findings indicate that warming exacerbates UV-B stress and severely limits recruitment (Müller, Wiencke & Bischof, 2008).

In a warming experiment studying Arctic populations of Saccharina latissima, no gametophytes survived at 20°C, but most growth parameters were greater at 10 to 15°C than at 5°C (Park et al., 2017). Another warming experiment involving Saccharina latissima from Kongsfjorden (Svalbard, Norway) highlighted an increase in physiological performance and growth in samples at 15°C (compared to 0°C), and that at least Arctic populations of Saccharina latissima can adjust and might even benefit from increased temperatures (Li et al., 2020). However, Gordillo, Carmona & Jimenez (2022) observed how Arctic individuals of Saccharina latissima lost more biomass in the dark at higher temperatures than lower ones, with a warmer polar night posing a limit on multi-year seaweeds to occupy new ice-free illuminated areas of the Arctic coasts.

Temperature is an environmental factor controlling the development of the microscopic stages of Saccharina latissima, with crucial changes in survival, growth, and gametogenesis occurring within a few degrees of its upper thermal limits (Redmond, 2013). The optimal germination temperature for Saccharina latissima is between 2°C and 12°C, with gametophyte survival between 23 to 25°C (Müller et al., 2009). Germination rates drop at 22°C, with surviving gametophytes smaller than those grown at lower temperatures (Redmond, 2013). Park et al. (2017) observed reductions in the percentage of sporophytes produced at 15°C when compared to values produced at 5°C and 10°C. Fales et al. (2023) compared the physiological responses of Saccharina latissima sporophytes to high temperature stress (low: 9 and 13°C, moderate: 15 and 16°C, and warm: 21°C) and nitrogen limitation (low: 1 to 3 μM vs. high: >10 μM) over 8 to 9 days. Saccharina latissima responded negatively to elevated temperatures, but not to low nitrogen levels. Blades of Saccharina latissima showed signs of metabolic stress and reduced growth in the warmest temperature treatment (21°C), at both high and low nitrogen levels, suggesting that Saccharina latissima is susceptible to thermal stress over short time periods, and that nutrient additions may actually reduce kelp performance at supra-optimal temperatures (Fales et al., 2023).

Niedzwiedz et al. (2022) also studied the response of Saccharina latissima sporophytes (sampled from Helgoland, German Bight, in June 2018, August 2018 and August 2019) to warming (at treatment temperatures of 18, 20, 22 and 24°C) and found that survival decreased with increasing environmental and experimental temperatures. Growth also revealed seasonal patterns, being higher in June than in August (Niedzwiedz et al., 2022). Niedzwiedz et al. (2022) concluded that the thermal tolerance of Saccharina latissima towards heatwaves in summer is significantly affected by the environmental history it previously experienced. This result has been seen in other experiments involving Saccharina latissima as well, whereby its sporophytes are pre-exposed to moderate stress to improve the performance and tolerance of plants when exposed to harsher conditions. This is known as thermal priming, and this may happen naturally as kelp are continually exposed to a warming climate. Gauci et al. (2024) observed how gametophytes primed at 20°C for four and six weeks exhibited an 11-day longer tolerance at 22°C, a seven-day longer tolerance at 23°C, and a 1°C higher thermal tolerance over seven days compared to two-week priming.

In the field, Saccharina latissima has shown significant regional variation in its acclimation response to changing environmental conditions. For example, Gerard & Dubois (1988) observed sporophytes of Saccharina latissima that were regularly exposed to ≥20°C tolerated these high temperatures, whereas sporophytes from other populations, which rarely experience ≥17°C, showed 100% mortality after three weeks of exposure to 20°C. At higher temperatures (11, 18 and 21°C), the nutritional content (C/N) of Saccharina latissima seems unaffected (Simonson, Scheibling & Metaxas, 2015b). However, the sea snail Lacuna vincta was observed grazing more kelp at higher temperatures (21°C) and suggests that the effects of grazing will act additively with the direct effects of temperature and cause increased biomass loss from kelp beds (Simonson, Scheibling & Metaxas, 2015b).

Saccharina latissima has suffered a dramatic decline in the Skagerrak region, Norway, where community structure has shifted from Saccharina latissima forests to communities dominated by filamentous macroalgae (Moy & Christie, 2012). In 2006, Andersen et al. (2011) transplanted Saccharina latissima into areas from where this species had been lost previously to determine whether the kelp could grow and mature. High mortality occurred from August to November each year. In 2008, only six of the seventeen original transplanted Saccharina latissima sporophytes survived (approx. 65% mortality rate). All surviving sporophytes were heavily fouled by epiphytic organisms (estimated cover of 80 & 100%). Between 1960 and 2009, sea surface temperatures in the region had regularly exceeded 20°C, and so had the duration at which temperatures remained above 20°C. High sea temperatures have been linked to the slow growth of Saccharina latissima, which is likely due to a decrease in the photosynthetic ability of Saccharina latissima, and an increase in vulnerability to epiphytic loading, bacterial and viral attacks (Anderson et al., 2011).

Kelp forests, including populations of Saccharina latissima, across the coastline of New England, USA, have experienced population shifts since the start of the 21^st century. Suskiewicz et al. (2024) surveyed between 31 and 67 forests spanning >350 km of coastline in Maine between 2001 and 2018 and then modelled how temperature change and sea urchin density influenced kelp abundance. Notably, the time-period studied was marked by rapid regional warming and several marine heatwaves, and the length of coastline examined experiences a more than 6°C difference in summer seawater temperatures from north to south (Suskiewicz et al., 2024). The maximum summer Near-Surface Seawater Temperatures in southern Maine commonly exceeded 20°C and were, on average, approx. 5.6°C warmer than those observed in northeast Maine (Suskiewicz et al., 2024). Consequently, southwestern subregions now regularly experience temperatures (15°C) at which nitrate saturation reaches zero (García-Reyes et al., 2022 and Zimmerman & Kremer, 1984 cited in Suskiewicz et al., 2024) as well as temperatures (20°C) at which sugar kelp erodes faster than it grows (Lee & Brinkhuis, 1986, cited in Suskiewicz et al., 2024). Also, high seawater temperatures reduce nutrient availability to kelp, causing nutrient depletion at 15°C (García-Reyes et al., 2022 and Zimmerman & Kremer, 1984 cited in Suskiewicz et al., 2024); and reduced nutrients during periods of maximum growth (spring) or thermal stress (summer) can accelerate kelp loss over time, as seen across all subregions by the end of the study by Suskiewicz et al. (2024). Although forests (Saccharina latissima and Laminaria digitata) had broadly returned to Maine in the late 20^th century, forests in northeast Maine have since experienced slow but significant declines in kelp, and forest persistence in the northeast was juxtaposed by a rapid, widespread collapse in the southwest (Suskiewicz et al., 2024). Forests collapsed in the southwest likely because ocean warming has directly and indirectly made this area inhospitable to kelp (Suskiewicz et al., 2024).

Hill et al. (2025) used species distribution models to evaluate the potential of enhanced thermal tolerance to buffer the effects of climate change (an increase of 1 to 5°C in maximum sea surface temperature) on cold-adapted kelp species. The models demonstrated that an increase of 1 to 2°C in thermal tolerance could recover over 50% of predicted losses of suitable habitat for cold-adapted kelps, with Saccharina latissima peaking at 17°C (Hill et al., 2025). For example, in the East Atlantic, Saccharina latissima recovery was concentrated in the southeast UK, but all species had projected patches of recovery on the Iberian coastline (Hill et al., 2025). In the North Sea and Skagerrak regions, a tolerance increase of 4 to 5°C was required for complete recovery (Hill et al., 2025). In the Baltic Sea, Saccharina latissima recovered with a tolerance increase of 1 to 2°C except for the mouth of the Baltic, where some areas remained unrecovered, even with a 5°C increase in tolerance (Hill et al., 2025). Overall, Saccharina latissima had the highest recovery potential with 99% of its projected lost suitable habitat area recovered under all climate change scenarios explored using the species distribution models (Hill et al., 2025). However, relying on mitigation or adaptation alone will likely be insufficient to maintain their historic range under projected climate change (Hill et al., 2025).

Similarly, Goldsmit et al. (2021) used a Random Forest model to predict future habitat suitability and cover for the dominant kelp species under climate change scenarios in the Eastern Canadian Arctic. Saccharina latissima is projected to have the largest gain in suitable habitat in both 2050 and 2100, with declines projected for some areas (e.g., north of Baffin Bay, Foxe Basin and Hudson Bay) by 2100 (Goldsmit et al., 2021). In general, suitable habitat is projected to occur in the northernmost reaches of the Eastern Canadian Arctic and is expected to persist into the future (Goldsmit et al., 2021). As the ocean warms and ice recedes, the model by Goldsmit et al. (2021) projects that Saccharina latissima will gain suitable habitat along much of the west coast of Greenland and the northern arm of the Northwest Passage.

Assis et al. (2018) predicted that, under the highest emission scenario (RCP 8.5), the range of Saccharina latissima would move northwards, retreating from the coast of Portugal, France and the southwest coast of the UK. The authors projected that, under RCP 2.6, 13% suitable Laminaria hyperborea habitat would be lost from the Western English Channel, while under the RCP 8.5 emission, 87% of suitable habitat was expected to be lost.

Chorda filum is a cold boreal species, with a wide geographical distribution along the Arctic, Atlantic and Pacific coasts (www.obis.org). Chorda filum has been reported to have relatively good growth between the temperatures of 5°C and 15°C but reduced or inhibited growth at 20°C (Kawai et al., 2000). Chorda filum has an upper temperature tolerance of 26-28°C (Dieck, 1993). Although Lüning (1980) observed that between the temperatures of 15 and 20°C, Chorda filum could not reproduce, but found that sporophytes could tolerate ≤26°C. In addition, Lüning (1990) reported gametogenesis to occur at temperatures between 5°C and 10°C in the autumn months. 

Wilson et al. (2015) reported that an increase in sea surface temperature from 1974 to 2010 resulted in biogeographical changes, with declines in abundance of Chorda filum, particularly in the English Channel. Wilson et al. (2015) suggested the declines of Chorda filum could be because the summer temperatures in those southern regions were too high for gametogenesis. 

Many of the red algae species associated with the understorey turf can tolerate warm water temperatures. 

Sensitivity Assessment. UK populations of Saccharina latissima are found in the middle of the species distribution and are known to be able to survive at higher temperatures than currently experienced around the UK. The ability to tolerate summer seawater temperatures of >20°C in populations at their southern geographic limit is thought to be a genetic adaptation (Gerard & Du Bois, 1988) and may be crucial in the persistence of this species around the UK, as seawater temperatures rise.

With sea surface temperature around the UK of between 6 and 19°C (Huthnance, 2010), populations of Saccharina latissima and the understorey community of mixed red seaweeds may be able to adapt to cope with a gradual rise in ocean temperatures of 3°C (middle emission scenario) by the end of this century, leading to maximum summer high temperatures in the south of the UK of 22°C. However, increasing temperatures are likely to lead to a decrease in growth and some mortality. Therefore, resistance is assessed as ‘Medium’, and resilience is assessed as ‘Very Low’, as the loss is likely to be a long-term decline, due to the long-term nature of ocean warming. Therefore, this biotope is assessed as ‘Medium’ sensitivity to ocean warming in the middle emission scenario.

For the high emission scenario and extreme scenario, whereby sea temperatures rise by 4-5°C to potential southern summer temperatures of 23-24°C by the end of this century, Saccharina latissima is likely to be lost from southern England, as gametophytes are not thought to be able to survive at temperatures ≥23°C. This assessment corresponds with the results of ecological niche modelling by Assis et al. (2018), who predicted that Saccharina latissima would be lost from the southwest coast of the UK because of climate change. Therefore, resistance is assessed as ‘Low’, and resilience is assessed as ‘Very low’. This biotope is assessed as having ‘High’ sensitivity to ocean warming in the high and extreme emission scenarios.

Marine heatwaves (high)

High emission scenario benchmark: A marine heatwave occurring every two years, with a mean duration of 120 days, and a maximum intensity of 3.5°C. Further detail.

Evidence

Marine heatwaves are extreme weather events defined as periods of extreme sea surface temperature that persist for days to months (Frölicher et al., 2018). Marine heatwaves are predicted to occur more frequently, last for longer and at increased intensity by the end of this century under both middle and high emission scenarios (Frölicher et al., 2018). Marine heatwaves are known to cause significant impacts to kelp forests, particularly if a population is found towards the edge of its southern limit (Smale et al., 2019). 

Saccharina latissima has disappeared almost completely from the Danish estuary Limfjorden, where maximum surface temperatures in summer have increased by 0.7°C per decade over the last 40 years, while the number of days with temperatures above 20°C has increased dramatically from 1-2 days per year to >25 days per year (Pedersen, 2015). Similarly, Saccharina latissima has been lost from the Skagerrak coast of Norway, which is thought to be due to an increase in summer temperatures, coupled with eutrophication (Moy & Christie, 2012).

Davey et al. (2025) studied the effect of short-term sublethal heat shock (20°C vs. ambient 10°C) on the health (growth and productivity), physiological performance (photosynthetic variables) and potential for compensatory mechanisms (phenolic content) of Saccharina latissima. The effect of heat shock was tested over five time points (0, 6, 24, 48, 72 hours). Growth of Saccharina latissima increased by 56% under heat shock, and gross primary productivity was initially greater in heat shock treatments (after six hours) but declined after 48 hours (Davey et al. 2025). Davey et al. (2025) concluded that Saccharina latissima exhibits the potential for short-term acclimation to sublethal heat shock, which may provide resistance to extreme temperature events, but that responses are species-specific.

Ding, Derksen & Timmermans (2025) investigated physiological and biochemical responses of juvenile Saccharina latissima sporophytes to acute (1-day to 10-day) and chronic (20-day to 40-day) warming from 11°C to 21°C, followed by exposure to 25°C. Acute warming (mimicking marine heatwaves) impaired physiological performance and reduced survival of juvenile Saccharina latissima sporophytes, whereas chronic warming led to elevated carbon and nitrogen reserves, increased fucoidan and protein levels, and enhanced photosynthetic performance. Improved heat tolerance of juvenile Saccharina latissima sporophytes was observed only in sporophytes previously exposed to 25°C, only after prior chronic warming treatments (Ding, Derksen & Timmermans, 2025). Ding, Derksen & Timmermans (2025) concluded that while exposure to chronic (gradual) temperature increases may allow Saccharina latissima to acclimate, events can exceed their physiological limits, leading to low survival, especially acute warming, which ultimately determines the presence or distribution of Saccharina latissima.

Under experimental conditions, Nepper-Davidsen, Andersen & Pedersen (2019) exposed a northern (Denmark) population of Saccharina latissima to a simulated three-week heatwave of three different intensities, 18, 21 and 24°C. When exposed to heatwaves of 18 and 21°C, there was a decrease in photosynthesis and growth. When 24°C was simulated, 91% of sporophytes were dead within a week, and the fronds of the few survivors were disintegrating, so the experiment was terminated (Nepper-Davidsen, Andersen & Pedersen et al., 2019). The results show that exposure to high, but sub-lethal, temperatures can have significant long-term effects, which may cause loss of biomass and leave Saccharina latissima susceptible to other stressors (Nepper-Davidsen, Andersen & Pedersen, 2019). This suggests that Saccharina latissima is unlikely to survive heatwaves of the length and magnitude predicted by the end of this century for both the middle and high emission scenarios.

Simonson, Scheibling & Metaxas (2015a) investigated the impacts of four temperature treatments (11, 14, 18 and 21°C) on growth, net length change and mortality of Saccharina latissima in Nova Scotia. Histological analysis showed temperature-mediated tissue damage, including holes, splitting of the medulla, damage to the meristoderm and loss of differentiation between tissue layers at temperatures between 14 and 21°C. Exposure to 21°C for one week reduced blade tissue strength (breaking stress) and extensibility (breaking strain) by 40 to 70% in and exhibited reduced strength after three-week exposure to 18°C (Simonson, Scheibling & Metaxas, 2015a). At the time, kelp species in Nova Scotia were experiencing large population declines, of which temperature could have been a contributing factor. However, Krumhansl et al. (2023) analysed the changes in Nova Scotia kelp abundance over the past 40 years (1982 to 2022) and found that there has been a loss in cold-tolerant kelps (such as Alaria esculenta, Saccorhiza dermatodea, and Agarum clathratum) and an increase in favour of the more warm-tolerant kelps like Saccharina latissima and Laminaria digitata. Kelp abundance increased since 2000, with Saccharina latissima widely abundant in the region by 2022 (Krumhansl et al., 2023). The highest kelp cover occurred on wave-exposed shores and at sites where temperatures have remained below thresholds for growth (21°C) and mortality (23°C) (Krumhansl et al., 2023). Moreover, kelp has recovered from turf dominance following losses at some sites during a warm period from 2010 to 2012 (Krumhansl et al., 2023). Krumhansl et al. (2023) concludes that that the dramatic change seen in kelp community composition in Nova Scotia over the past 40 years is in part driven by the loss of sea urchin herbivory, but a broad-scale shift to turf-dominance has not occurred, and that resilience and persistence are still a feature of kelp forests in the region despite rapid warming over the past several decades.

Miller et al. (2024a) conducted a 23-day mesocosm experiment exposing mixed kelp communities to warming and heatwave scenarios projected for the year 2100 to assess their impact. Three treatments were considered: a constant warming (+1.8°C from the control), a medium magnitude and long duration heatwave event (+2.8°C from the control for 13 days), and two short-term, more intense, heatwaves (5-day long scenarios with temperature peaks at +3.9°C from the control). The results showed that both marine heatwave treatments reduced net community production, whereas the constant warm temperature treatment displayed no difference from the control (Miller et al., 2024a). The long marine heatwave scenario resulted in reduced accumulated net community production, indicating that prolonged exposure had a greater severity than two high-magnitude, short-term heatwave events (Miller et al., 2024a). Miller et al. (2024a) estimated an 11°C temperature threshold at which negative effects to primary production appeared present, and that marine heatwaves can induce sublethal effects on kelp communities by depressing net community production.

In southern Norway, the frequency and intensity of marine heatwaves have been correlated to decreasing kelp biomass resulting from an increasing trend in the duration of temperature anomalies at a rate of 0.17 days/year over the past 60 years (Filbee-Dexter et al., 2020 cited in Miller et al., 2024a). This leads to temperatures surpassing the mortality threshold of 19.7°C for populations of Saccharina latissima (Miller et al., 2024a). During the mesocosm experiment by Miller et al. (2024a), temperatures reached a maximum of approx. 14°C, which should be below the mortality threshold, although the populations studied are from an Arctic fjord in northern Norway. This is particularly relevant given that ecotypes may demonstrate a difference in temperature tolerance (King et al., 2019 cited in Miller et al., 2024a) and the acclimatization potential to marine heatwaves by specific ecotypes (Miller et al., 2024a). It is important to note that the negative effects of marine heatwaves presented here do not indicate mass mortality or significant senescence, but more of a sublethal effect on community production (Miller et al., 2024a).

Interestingly, Saccharina latissima has been shown to potentially carry a thermal history where exposure to a previously high temperature anomaly, or its accumulated exposure duration reduces its tolerance to future anomalies (Niedzwiedz et al., 2022). This result has been seen in other experiments involving Saccharina latissima as well, whereby its sporophytes are pre-exposed to moderate stress to improve the performance and tolerance of plants when exposed to harsher conditions. This is known as thermal priming, and this may happen naturally as kelp are continually exposed to a warming climate. Gauci et al. (2024) observed how gametophytes primed at 20°C for four and six weeks exhibited an 11-day longer tolerance at 22°C, a seven-day longer tolerance at 23°C, and a 1°C higher thermal tolerance over seven days compared to two-week priming.

Kelp forests, including populations of Saccharina latissima, across the coastline of New England, USA, have experienced population shifts since the start of the 21^st century. Suskiewicz et al. (2024) surveyed between 31 and 67 forests spanning >350 km of coastline in Maine between 2001 and 2018 and then modelled how temperature change and sea urchin density influenced kelp abundance. Notably, the time-period studied was marked by rapid regional warming and several marine heatwaves, and the length of coastline examined experiences a more than 6°C difference in summer seawater temperatures from north to south (Suskiewicz et al., 2024). Maximum summer Near-Surface Seawater Temperatures in southern Maine commonly exceeded 20°C and were, on average, ~5.6°C warmer than those observed in northeast Maine (Suskiewicz et al., 2024). Consequently, southwestern subregions now regularly experience temperatures (15°C) at which nitrate saturation reaches zero (García-Reyes et al., 2022 and Zimmerman & Kremer, 1984 cited in Suskiewicz et al., 2024) as well as temperatures (20°C) at which sugar kelp erodes faster than it grows (Lee & Brinkhuis, 1986 cited in Suskiewicz et al., 2024). Also, high seawater temperatures reduce nutrient availability to kelp, causing nutrient depletion at 15°C (García-Reyes et al., 2022 and Zimmerman & Kremer, 1984 cited in Suskiewicz et al., 2024); and reduced nutrients during periods of maximum growth (spring) or thermal stress (summer) can accelerate kelp loss over time, as seen across all subregions by the end of the study by Suskiewicz et al. (2024). Although forests (Saccharina latissima and Laminaria digitata) had broadly returned to Maine in the late 20^th century, forests in northeast Maine have since experienced slow but significant declines in kelp, and forest persistence in the northeast was juxtaposed by a rapid, widespread collapse in the southwest (Suskiewicz et al., 2024). Forests collapsed in the southwest likely because ocean warming has directly and indirectly made this area inhospitable to kelp (Suskiewicz et al., 2024).

Sensitivity Assessment. Under the middle emission scenario, if heatwaves occurred every three years, with a maximum intensity of 2°C for 80 days by the end of this century, this could lead to summer sea temperatures reaching up to 24°C in southern England. A heatwave of this magnitude is likely to cause mass mortality of Saccharina latissima. Therefore, resistance has been assessed as ‘None’. As widespread mortality may lead to a lack of viable sporophytes for recruitment, resilience has been assessed as ‘Very low.’ This biotope is assessed as having ‘High’ sensitivity to marine heatwaves under the middle emission scenario.

Under the high emission scenario, if heatwaves occur every two years by the end of this century, reaching a maximum intensity of 3.5°C for 120 days, this could lead to the heatwave lasting the entire summer with temperatures reaching up to 26.5°C. Under this scenario, Saccharina latissima is likely to be already lost from this biotope as a result of rising temperatures (see Global warming), although mortality of any surviving specimens would occur as a result of this projected heatwave. Therefore, resistance has been assessed as ‘None’. As widespread mortality may lead to a lack of viable sporophytes for recruitment, resilience has been assessed as ‘Very low.’ Therefore, this biotope is assessed as having ‘High’ sensitivity to marine heatwaves under the high emission scenario.

Marine heatwaves (middle)

Middle emission scenario benchmark:  A marine heatwave occurring every three years, with a mean duration of 80 days, with a maximum intensity of 2°C. Further detail.

Evidence

Marine heatwaves are extreme weather events defined as periods of extreme sea surface temperature that persist for days to months (Frölicher et al., 2018). Marine heatwaves are predicted to occur more frequently, last for longer and at increased intensity by the end of this century under both middle and high emission scenarios (Frölicher et al., 2018). Marine heatwaves are known to cause significant impacts to kelp forests, particularly if a population is found towards the edge of its southern limit (Smale et al., 2019). 

Saccharina latissima has disappeared almost completely from the Danish estuary Limfjorden, where maximum surface temperatures in summer have increased by 0.7°C per decade over the last 40 years, while the number of days with temperatures above 20°C has increased dramatically from 1-2 days per year to >25 days per year (Pedersen, 2015). Similarly, Saccharina latissima has been lost from the Skagerrak coast of Norway, which is thought to be due to an increase in summer temperatures, coupled with eutrophication (Moy & Christie, 2012).

Davey et al. (2025) studied the effect of short-term sublethal heat shock (20°C vs. ambient 10°C) on the health (growth and productivity), physiological performance (photosynthetic variables) and potential for compensatory mechanisms (phenolic content) of Saccharina latissima. The effect of heat shock was tested over five time points (0, 6, 24, 48, 72 hours). Growth of Saccharina latissima increased by 56% under heat shock, and gross primary productivity was initially greater in heat shock treatments (after six hours) but declined after 48 hours (Davey et al. 2025). Davey et al. (2025) concluded that Saccharina latissima exhibits the potential for short-term acclimation to sublethal heat shock, which may provide resistance to extreme temperature events, but that responses are species-specific.

Ding, Derksen & Timmermans (2025) investigated physiological and biochemical responses of juvenile Saccharina latissima sporophytes to acute (1-day to 10-day) and chronic (20-day to 40-day) warming from 11°C to 21°C, followed by exposure to 25°C. Acute warming (mimicking marine heatwaves) impaired physiological performance and reduced survival of juvenile Saccharina latissima sporophytes, whereas chronic warming led to elevated carbon and nitrogen reserves, increased fucoidan and protein levels, and enhanced photosynthetic performance. Improved heat tolerance of juvenile Saccharina latissima sporophytes was observed only in sporophytes previously exposed to 25°C, only after prior chronic warming treatments (Ding, Derksen & Timmermans, 2025). Ding, Derksen & Timmermans (2025) concluded that while exposure to chronic (gradual) temperature increases may allow Saccharina latissima to acclimate, events can exceed their physiological limits, leading to low survival, especially acute warming, which ultimately determines the presence or distribution of Saccharina latissima.

Under experimental conditions, Nepper-Davidsen, Andersen & Pedersen (2019) exposed a northern (Denmark) population of Saccharina latissima to a simulated three-week heatwave of three different intensities, 18, 21 and 24°C. When exposed to heatwaves of 18 and 21°C, there was a decrease in photosynthesis and growth. When 24°C was simulated, 91% of sporophytes were dead within a week, and the fronds of the few survivors were disintegrating, so the experiment was terminated (Nepper-Davidsen, Andersen & Pedersen et al., 2019). The results show that exposure to high, but sub-lethal, temperatures can have significant long-term effects, which may cause loss of biomass and leave Saccharina latissima susceptible to other stressors (Nepper-Davidsen, Andersen & Pedersen, 2019). This suggests that Saccharina latissima is unlikely to survive heatwaves of the length and magnitude predicted by the end of this century for both the middle and high emission scenarios.

Simonson, Scheibling & Metaxas (2015a) investigated the impacts of four temperature treatments (11, 14, 18 and 21°C) on growth, net length change and mortality of Saccharina latissima in Nova Scotia. Histological analysis showed temperature-mediated tissue damage, including holes, splitting of the medulla, damage to the meristoderm and loss of differentiation between tissue layers at temperatures between 14 and 21°C. Exposure to 21°C for one week reduced blade tissue strength (breaking stress) and extensibility (breaking strain) by 40 to 70% in and exhibited reduced strength after three-week exposure to 18°C (Simonson, Scheibling & Metaxas, 2015a). At the time, kelp species in Nova Scotia were experiencing large population declines, of which temperature could have been a contributing factor. However, Krumhansl et al. (2023) analysed the changes in Nova Scotia kelp abundance over the past 40 years (1982 to 2022) and found that there has been a loss in cold-tolerant kelps (such as Alaria esculenta, Saccorhiza dermatodea, and Agarum clathratum) and an increase in favour of the more warm-tolerant kelps like Saccharina latissima and Laminaria digitata. Kelp abundance increased since 2000, with Saccharina latissima widely abundant in the region by 2022 (Krumhansl et al., 2023). The highest kelp cover occurred on wave-exposed shores and at sites where temperatures have remained below thresholds for growth (21°C) and mortality (23°C) (Krumhansl et al., 2023). Moreover, kelp has recovered from turf dominance following losses at some sites during a warm period from 2010 to 2012 (Krumhansl et al., 2023). Krumhansl et al. (2023) concludes that that the dramatic change seen in kelp community composition in Nova Scotia over the past 40 years is in part driven by the loss of sea urchin herbivory, but a broad-scale shift to turf-dominance has not occurred, and that resilience and persistence are still a feature of kelp forests in the region despite rapid warming over the past several decades.

Miller et al. (2024a) conducted a 23-day mesocosm experiment exposing mixed kelp communities to warming and heatwave scenarios projected for the year 2100 to assess their impact. Three treatments were considered: a constant warming (+1.8°C from the control), a medium magnitude and long duration heatwave event (+2.8°C from the control for 13 days), and two short-term, more intense, heatwaves (5-day long scenarios with temperature peaks at +3.9°C from the control). The results showed that both marine heatwave treatments reduced net community production, whereas the constant warm temperature treatment displayed no difference from the control (Miller et al., 2024a). The long marine heatwave scenario resulted in reduced accumulated net community production, indicating that prolonged exposure had a greater severity than two high-magnitude, short-term heatwave events (Miller et al., 2024a). Miller et al. (2024a) estimated an 11°C temperature threshold at which negative effects to primary production appeared present, and that marine heatwaves can induce sublethal effects on kelp communities by depressing net community production.

In southern Norway, the frequency and intensity of marine heatwaves have been correlated to decreasing kelp biomass resulting from an increasing trend in the duration of temperature anomalies at a rate of 0.17 days/year over the past 60 years (Filbee-Dexter et al., 2020 cited in Miller et al., 2024a). This leads to temperatures surpassing the mortality threshold of 19.7°C for populations of Saccharina latissima (Miller et al., 2024a). During the mesocosm experiment by Miller et al. (2024a), temperatures reached a maximum of approx. 14°C, which should be below the mortality threshold, although the populations studied are from an Arctic fjord in northern Norway. This is particularly relevant given that ecotypes may demonstrate a difference in temperature tolerance (King et al., 2019 cited in Miller et al., 2024a) and the acclimatization potential to marine heatwaves by specific ecotypes (Miller et al., 2024a). It is important to note that the negative effects of marine heatwaves presented here do not indicate mass mortality or significant senescence, but more of a sublethal effect on community production (Miller et al., 2024a).

Interestingly, Saccharina latissima has been shown to potentially carry a thermal history where exposure to a previously high temperature anomaly, or its accumulated exposure duration reduces its tolerance to future anomalies (Niedzwiedz et al., 2022). This result has been seen in other experiments involving Saccharina latissima as well, whereby its sporophytes are pre-exposed to moderate stress to improve the performance and tolerance of plants when exposed to harsher conditions. This is known as thermal priming, and this may happen naturally as kelp are continually exposed to a warming climate. Gauci et al. (2024) observed how gametophytes primed at 20°C for four and six weeks exhibited an 11-day longer tolerance at 22°C, a seven-day longer tolerance at 23°C, and a 1°C higher thermal tolerance over seven days compared to two-week priming.

Kelp forests, including populations of Saccharina latissima, across the coastline of New England, USA, have experienced population shifts since the start of the 21^st century. Suskiewicz et al. (2024) surveyed between 31 and 67 forests spanning >350 km of coastline in Maine between 2001 and 2018 and then modelled how temperature change and sea urchin density influenced kelp abundance. Notably, the time-period studied was marked by rapid regional warming and several marine heatwaves, and the length of coastline examined experiences a more than 6°C difference in summer seawater temperatures from north to south (Suskiewicz et al., 2024). Maximum summer Near-Surface Seawater Temperatures in southern Maine commonly exceeded 20°C and were, on average, ~5.6°C warmer than those observed in northeast Maine (Suskiewicz et al., 2024). Consequently, southwestern subregions now regularly experience temperatures (15°C) at which nitrate saturation reaches zero (García-Reyes et al., 2022 and Zimmerman & Kremer, 1984 cited in Suskiewicz et al., 2024) as well as temperatures (20°C) at which sugar kelp erodes faster than it grows (Lee & Brinkhuis, 1986 cited in Suskiewicz et al., 2024). Also, high seawater temperatures reduce nutrient availability to kelp, causing nutrient depletion at 15°C (García-Reyes et al., 2022 and Zimmerman & Kremer, 1984 cited in Suskiewicz et al., 2024); and reduced nutrients during periods of maximum growth (spring) or thermal stress (summer) can accelerate kelp loss over time, as seen across all subregions by the end of the study by Suskiewicz et al. (2024). Although forests (Saccharina latissima and Laminaria digitata) had broadly returned to Maine in the late 20^th century, forests in northeast Maine have since experienced slow but significant declines in kelp, and forest persistence in the northeast was juxtaposed by a rapid, widespread collapse in the southwest (Suskiewicz et al., 2024). Forests collapsed in the southwest likely because ocean warming has directly and indirectly made this area inhospitable to kelp (Suskiewicz et al., 2024).

Sensitivity Assessment. Under the middle emission scenario, if heatwaves occurred every three years, with a maximum intensity of 2°C for 80 days by the end of this century, this could lead to summer sea temperatures reaching up to 24°C in southern England. A heatwave of this magnitude is likely to cause mass mortality of Saccharina latissima. Therefore, resistance has been assessed as ‘None’. As widespread mortality may lead to a lack of viable sporophytes for recruitment, resilience has been assessed as ‘Very low.’ This biotope is assessed as having ‘High’ sensitivity to marine heatwaves under the middle emission scenario.

Under the high emission scenario, if heatwaves occur every two years by the end of this century, reaching a maximum intensity of 3.5°C for 120 days, this could lead to the heatwave lasting the entire summer with temperatures reaching up to 26.5°C. Under this scenario, Saccharina latissima is likely to be already lost from this biotope as a result of rising temperatures (see Global warming), although mortality of any surviving specimens would occur as a result of this projected heatwave. Therefore, resistance has been assessed as ‘None’. As widespread mortality may lead to a lack of viable sporophytes for recruitment, resilience has been assessed as ‘Very low.’ Therefore, this biotope is assessed as having ‘High’ sensitivity to marine heatwaves under the high emission scenario.

Ocean acidification (high)

High emission scenario benchmark: a further decrease in pH of 0.35 (annual mean) and corresponding 120% increase in H+ ions , seasonal aragonite saturation of 20% of UK coastal waters and North Sea bottom waters, and the aragonite saturation horizon in the NE Atlantic, off the continental shelf, occurring at a depth of 400 m by the end of this century 2081-2100. Further detail 

Evidence

Increasing levels of CO₂ in the atmosphere have led to the average pH of sea surface waters dropping from 8.25 in the 1700s to 8.14 in the 1990s (Jacobson, 2005), with it expected to drop up to a further 0.35 units by the end of this century, dependent on emission scenario (Meehl et al., 2017 cited in Kerrison et al., 2015). Marine autotrophs will generally benefit from ocean acidification through an increase in the availability of aqueous CO₂ for photosynthesis (Koch et al., 2013), however, no clear conclusion can be made about the response of kelps, since macroalgal responses appear to be highly species-specific (Zou & Gao, 2010 cited in Kerrison et al., 2015). Research on most kelp species has revealed a positive or neutral effect of ocean acidification (Roleda et al., 2012; Fernández et al., 2015; Nunes et al., 2016; Iñiguez et al., 2016b, a), except for one study, which found that ocean acidification negatively impacted photosynthesis and growth in the southern hemisphere species, Ecklonia radiata (Britton et al., 2016).

Saccharina latissima has a pH compensation point of 9.6 to 9.8 indicating the presence of an effective carbon concentrating mechanism (Maberly, 1990 cited in Kerrison et al., 2015), with some experiments showing that large leathery macroalgae, appear to cope well in areas of naturally low pH (Hall-Spencer et al., 2008 and Porzio, Buia & Hall-Spencer, 2011 cited in Kerrison et al., 2015), while others show a decreased growth of Saccharina latissima at a lower pH (Swanson & Fox, 2007 cited in Kerrison et al., 2015).

Under experimental CO₂ enrichment at levels expected by the end of this century, germination rates in Saccharina latissima were the same as control samples, but gametophyte size increased, suggesting a benefit for juvenile stages of this species (Roleda et al., 2012). Nunes et al. (2016) found that experimental exposure of adult Saccharina latissima to enhanced CO₂ led to an increase in net primary production, while Gordillo et al. (2015) found that enhanced CO₂ led to increased photosynthesis and growth. In contrast, Iñiguez et al. (2016b) found no increase in carbon fixation under elevated CO₂ conditions. Although contrasting in findings, these studies show that ocean acidification will not negatively impact Saccharina latissima.

Young, Doall & Gobler (2021) observed the effect of acidification on Saccharina latissima alongside changes in nutrients and grazing by the gastropod Lacuna vincta. They noted how under elevated nutrients, Saccharina latissima experienced significantly enhanced growth at pCO₂ levels ≥1200 µatm compared to ambient pCO₂ (~400 µatm); in addition, elevated pCO₂ (≥830 µatm) also significantly reduced herbivory of Lacuna vincta grazing on Saccharina latissima relative to ambient pCO₂ (Young, Doall & Gobler, 2021). Decreased herbivory was specifically elicited when Lacuna vincta were exposed to elevated pCO₂ in the absence of food for ≥18 hours prior to grazing, with reduced grazing persisting 72 hours (Young, Doall & Gobler, 2021). Elevated growth of Saccharina latissima and reduced grazing by Lacuna vincta at 1200 µatm pCO₂ combined to increase net growth rates of Saccharina latissima more than four-fold relative to ambient pCO₂ (Young, Doall & Gobler, 2021). Lacuna vincta consumed 70% of daily production by Saccharina latissima under ambient pCO₂ but only 38 and 9% at 800 and 1200 µatm, respectively (Young, Doall & Gobler, 2021). Young, Doall & Gobler (2021) concluded that decreased grazing by Lacuna vincta coupled with enhanced growth of Saccharina latissima under elevated pCO₂ demonstrates that increased CO₂ associated with climate change and/or coastal processes will dually benefit commercially and ecologically important kelps by both promoting growth and reducing grazing pressure.

Sensitivity Assessment. Kelp forests live in a naturally variable pH habitat, with diel fluctuations of 0.3 to 0.45 pH units (Krause-Jensen et al., 2015; Britton et al., 2016), and boundary layer pH fluctuation of up to 0.8 units (Krause-Jensen et al., 2015). Saccharina latissima is not expected to exhibit negative effects from ocean acidification at levels expected for the end of this century. Due to the disturbed nature of the biotope, the understorey community can vary locally, therefore impacts to the understorey community have not been included in the assessment. Under both the middle and high emission scenarios, resistance is assessed as ‘High’, and resilience is assessed as ‘High’, so that sensitivity is assessed as ‘Not sensitive’.

Ocean acidification (middle)

Middle emission scenario benchmark: a further decrease in pH of 0.15 (annual mean) and corresponding 35% increase in H+ ions with no coastal aragonite undersaturation and the aragonite saturation horizon in the NE Atlantic, off the continental shelf, at a depth of 800 m by the end of this century 2081-2100. Further detail.

Evidence

Increasing levels of CO₂ in the atmosphere have led to the average pH of sea surface waters dropping from 8.25 in the 1700s to 8.14 in the 1990s (Jacobson, 2005), with it expected to drop up to a further 0.35 units by the end of this century, dependent on emission scenario (Meehl et al., 2017 cited in Kerrison et al., 2015). Marine autotrophs will generally benefit from ocean acidification through an increase in the availability of aqueous CO₂ for photosynthesis (Koch et al., 2013), however, no clear conclusion can be made about the response of kelps, since macroalgal responses appear to be highly species-specific (Zou & Gao, 2010 cited in Kerrison et al., 2015). Research on most kelp species has revealed a positive or neutral effect of ocean acidification (Roleda et al., 2012; Fernández et al., 2015; Nunes et al., 2016; Iñiguez et al., 2016b, a), except for one study, which found that ocean acidification negatively impacted photosynthesis and growth in the southern hemisphere species, Ecklonia radiata (Britton et al., 2016).

Saccharina latissima has a pH compensation point of 9.6 to 9.8 indicating the presence of an effective carbon concentrating mechanism (Maberly, 1990 cited in Kerrison et al., 2015), with some experiments showing that large leathery macroalgae, appear to cope well in areas of naturally low pH (Hall-Spencer et al., 2008 and Porzio, Buia & Hall-Spencer, 2011 cited in Kerrison et al., 2015), while others show a decreased growth of Saccharina latissima at a lower pH (Swanson & Fox, 2007 cited in Kerrison et al., 2015).

Under experimental CO₂ enrichment at levels expected by the end of this century, germination rates in Saccharina latissima were the same as control samples, but gametophyte size increased, suggesting a benefit for juvenile stages of this species (Roleda et al., 2012). Nunes et al. (2016) found that experimental exposure of adult Saccharina latissima to enhanced CO₂ led to an increase in net primary production, while Gordillo et al. (2015) found that enhanced CO₂ led to increased photosynthesis and growth. In contrast, Iñiguez et al. (2016b) found no increase in carbon fixation under elevated CO₂ conditions. Although contrasting in findings, these studies show that ocean acidification will not negatively impact Saccharina latissima.

Young, Doall & Gobler (2021) observed the effect of acidification on Saccharina latissima alongside changes in nutrients and grazing by the gastropod Lacuna vincta. They noted how under elevated nutrients, Saccharina latissima experienced significantly enhanced growth at pCO₂ levels ≥1200 µatm compared to ambient pCO₂ (~400 µatm); in addition, elevated pCO₂ (≥830 µatm) also significantly reduced herbivory of Lacuna vincta grazing on Saccharina latissima relative to ambient pCO₂ (Young, Doall & Gobler, 2021). Decreased herbivory was specifically elicited when Lacuna vincta were exposed to elevated pCO₂ in the absence of food for ≥18 hours prior to grazing, with reduced grazing persisting 72 hours (Young, Doall & Gobler, 2021). Elevated growth of Saccharina latissima and reduced grazing by Lacuna vincta at 1200 µatm pCO₂ combined to increase net growth rates of Saccharina latissima more than four-fold relative to ambient pCO₂ (Young, Doall & Gobler, 2021). Lacuna vincta consumed 70% of daily production by Saccharina latissima under ambient pCO₂ but only 38 and 9% at 800 and 1200 µatm, respectively (Young, Doall & Gobler, 2021). Young, Doall & Gobler (2021) concluded that decreased grazing by Lacuna vincta coupled with enhanced growth of Saccharina latissima under elevated pCO₂ demonstrates that increased CO₂ associated with climate change and/or coastal processes will dually benefit commercially and ecologically important kelps by both promoting growth and reducing grazing pressure.

Sensitivity Assessment. Kelp forests live in a naturally variable pH habitat, with diel fluctuations of 0.3 to 0.45 pH units (Krause-Jensen et al., 2015; Britton et al., 2016), and boundary layer pH fluctuation of up to 0.8 units (Krause-Jensen et al., 2015). Saccharina latissima is not expected to exhibit negative effects from ocean acidification at levels expected for the end of this century. Due to the disturbed nature of the biotope, the understorey community can vary locally, therefore impacts to the understorey community have not been included in the assessment. Under both the middle and high emission scenarios, resistance is assessed as ‘High’, and resilience is assessed as ‘High’, so that sensitivity is assessed as ‘Not sensitive’.

Sea level rise (extreme)

Extreme scenario benchmark: a 107 cm rise in average UK by the end of this century (2018-2100). Further detail.

Evidence

Sea-level rise is occurring through a combination of thermal expansion and ice melt.  Sea levels have risen 1-3 mm/yr. in the last century (Cazenave & Nerem, 2004, Church et al., 2004, Church & White, 2006). Sea-level rise is expected to lead to substantial loss of intertidal habitats. Rocky shores backed by cliffs constitute about 80% of oceanic coastlines globally and in Britain, 42% of the coastline is hard rock, with many areas having cliffs behind the shore (Jackson & McIlvenny, 2011).

Light availability and water turbidity are principal factors in determining kelp depth range (Birkett et al. 1998b), with laminarians being reported to be able to withstand light levels of up to 1% surface irradiance. In Maine, USA, Saccharina latissima is abundant at both turbid and deep sites where surface irradiance averages 2.5% surface irradiance and have adapted to low-light conditions (Gerard, 1990).

This biotope (SS.SMp.KSwSS.SlatCho) occurs on sheltered, very sheltered and extremely sheltered infralittoral sediments from 0-10m (JNCC, 2015). Understanding how sea-level rise will affect tidal energy is fraught with uncertainty, although evidence appears to suggest that any alterations will be non-linear (Pickering et al., 2012, Li et al., 2016). Modelling potential outcomes of sea-level rise on the tidal and residual currents in the Bohai Sea, China showed effects were site-dependent, with energy either increasing or decreasing (Li et al., 2016). Similarly, Pickering et al. (2012) found a similar pattern around the UK for tidal amplitude. 

Saccharina latissima occurs in a wide range of water flow rates, from strong tidal currents to areas with low wave exposure (Birkett et al., 1998b). Therefore, Saccharina latissima is unlikely to be affected by a change in water flow. 

Chorda filum sporophytes often grow on unstable objects, such as pebbles and shell. Owing to the typically unstable substratum on which Chorda filum grows, whole populations can be moved during storms and deposited in more sheltered locations where development will continue (South & Burrows, 1967). A large increase in near-shore wave height is likely to significantly influence biotope structure. As highlighted by Connor et al. (2004), sub-biotopes within SS.SMp.KSwSS.SlatR are largely distinguished by wave exposure

Sensitivity assessment.  The biotope is recorded from 0 to 10 m in depth (JNCC, 2015). This biotope may be able to expand its range and migrate landwards to compensate for sea-level rise, if not constrained by lack of tide-swept rock, or human-modified shorelines (IPCC, 2019). If landward migration is not possible, it is expected that depth distribution of this biotope will shrink substantially in response to a 50, 70 or 107 cm sea-level rise, without the possibility of recovery, due to the increased depth, leading to a reduction in light availability for photosynthesis. 

There is likely to be considerable variation between sites, the relative contribution of wave surge and exposure to habitat suitability, and the depth range occupied by the biotope. Hence, it is difficult to assess the effect of the different sea-level rise scenarios. However, as the biotope can occur from 0-10 m in depth, it is assumed at a sea-level rise of 50 cm, or 70 cm (middle to high emission scenarios) would have limited effect but that a 107 cm rise (the extreme emission scenario) might result in loss of some of the deeper extent of the biotope in some sites. Therefore, resistance is assessed as ‘High’ under the middle and high emission scenarios so that resilience is ‘High’ and sensitivity assessed as ‘Not sensitive’. But resistance may be ‘Medium’ under the extreme emission scenario so that resilience is ‘Very low’ and sensitivity assessed as ‘Medium’, albeit with ‘Low’ confidence.

Sea level rise (high)

High emission scenario benchmark: a 70 cm rise in average UK by the end of this century (2018-2100). Further detail.

Evidence

Sea-level rise is occurring through a combination of thermal expansion and ice melt.  Sea levels have risen 1-3 mm/yr. in the last century (Cazenave & Nerem, 2004, Church et al., 2004, Church & White, 2006). Sea-level rise is expected to lead to substantial loss of intertidal habitats. Rocky shores backed by cliffs constitute about 80% of oceanic coastlines globally and in Britain, 42% of the coastline is hard rock, with many areas having cliffs behind the shore (Jackson & McIlvenny, 2011).

Light availability and water turbidity are principal factors in determining kelp depth range (Birkett et al. 1998b), with laminarians being reported to be able to withstand light levels of up to 1% surface irradiance. In Maine, USA, Saccharina latissima is abundant at both turbid and deep sites where surface irradiance averages 2.5% surface irradiance and have adapted to low-light conditions (Gerard, 1990).

This biotope (SS.SMp.KSwSS.SlatCho) occurs on sheltered, very sheltered and extremely sheltered infralittoral sediments from 0-10m (JNCC, 2015). Understanding how sea-level rise will affect tidal energy is fraught with uncertainty, although evidence appears to suggest that any alterations will be non-linear (Pickering et al., 2012, Li et al., 2016). Modelling potential outcomes of sea-level rise on the tidal and residual currents in the Bohai Sea, China showed effects were site-dependent, with energy either increasing or decreasing (Li et al., 2016). Similarly, Pickering et al. (2012) found a similar pattern around the UK for tidal amplitude. 

Saccharina latissima occurs in a wide range of water flow rates, from strong tidal currents to areas with low wave exposure (Birkett et al., 1998b). Therefore, Saccharina latissima is unlikely to be affected by a change in water flow. 

Chorda filum sporophytes often grow on unstable objects, such as pebbles and shell. Owing to the typically unstable substratum on which Chorda filum grows, whole populations can be moved during storms and deposited in more sheltered locations where development will continue (South & Burrows, 1967). A large increase in near-shore wave height is likely to significantly influence biotope structure. As highlighted by Connor et al. (2004), sub-biotopes within SS.SMp.KSwSS.SlatR are largely distinguished by wave exposure

Sensitivity assessment.  The biotope is recorded from 0 to 10 m in depth (JNCC, 2015). This biotope may be able to expand its range and migrate landwards to compensate for sea-level rise, if not constrained by lack of tide-swept rock, or human-modified shorelines (IPCC, 2019). If landward migration is not possible, it is expected that depth distribution of this biotope will shrink substantially in response to a 50, 70 or 107 cm sea-level rise, without the possibility of recovery, due to the increased depth, leading to a reduction in light availability for photosynthesis. 

There is likely to be considerable variation between sites, the relative contribution of wave surge and exposure to habitat suitability, and the depth range occupied by the biotope. Hence, it is difficult to assess the effect of the different sea-level rise scenarios. However, as the biotope can occur from 0-10 m in depth, it is assumed at a sea-level rise of 50 cm, or 70 cm (middle to high emission scenarios) would have limited effect but that a 107 cm rise (the extreme emission scenario) might result in loss of some of the deeper extent of the biotope in some sites. Therefore, resistance is assessed as ‘High’ under the middle and high emission scenarios so that resilience is ‘High’ and sensitivity assessed as ‘Not sensitive’. But resistance may be ‘Medium’ under the extreme emission scenario so that resilience is ‘Very low’ and sensitivity assessed as ‘Medium’, albeit with ‘Low’ confidence.

Sea level rise (middle)

Middle emission scenario benchmark: a 50 cm rise in average UK sea-level rise by the end of this century (2081-2100). Further detail.

Evidence

Sea-level rise is occurring through a combination of thermal expansion and ice melt.  Sea levels have risen 1-3 mm/yr. in the last century (Cazenave & Nerem, 2004, Church et al., 2004, Church & White, 2006). Sea-level rise is expected to lead to substantial loss of intertidal habitats. Rocky shores backed by cliffs constitute about 80% of oceanic coastlines globally and in Britain, 42% of the coastline is hard rock, with many areas having cliffs behind the shore (Jackson & McIlvenny, 2011).

Light availability and water turbidity are principal factors in determining kelp depth range (Birkett et al. 1998b), with laminarians being reported to be able to withstand light levels of up to 1% surface irradiance. In Maine, USA, Saccharina latissima is abundant at both turbid and deep sites where surface irradiance averages 2.5% surface irradiance and have adapted to low-light conditions (Gerard, 1990).

This biotope (SS.SMp.KSwSS.SlatCho) occurs on sheltered, very sheltered and extremely sheltered infralittoral sediments from 0-10m (JNCC, 2015). Understanding how sea-level rise will affect tidal energy is fraught with uncertainty, although evidence appears to suggest that any alterations will be non-linear (Pickering et al., 2012, Li et al., 2016). Modelling potential outcomes of sea-level rise on the tidal and residual currents in the Bohai Sea, China showed effects were site-dependent, with energy either increasing or decreasing (Li et al., 2016). Similarly, Pickering et al. (2012) found a similar pattern around the UK for tidal amplitude. 

Saccharina latissima occurs in a wide range of water flow rates, from strong tidal currents to areas with low wave exposure (Birkett et al., 1998b). Therefore, Saccharina latissima is unlikely to be affected by a change in water flow. 

Chorda filum sporophytes often grow on unstable objects, such as pebbles and shell. Owing to the typically unstable substratum on which Chorda filum grows, whole populations can be moved during storms and deposited in more sheltered locations where development will continue (South & Burrows, 1967). A large increase in near-shore wave height is likely to significantly influence biotope structure. As highlighted by Connor et al. (2004), sub-biotopes within SS.SMp.KSwSS.SlatR are largely distinguished by wave exposure

Sensitivity assessment.  The biotope is recorded from 0 to 10 m in depth (JNCC, 2015). This biotope may be able to expand its range and migrate landwards to compensate for sea-level rise, if not constrained by lack of tide-swept rock, or human-modified shorelines (IPCC, 2019). If landward migration is not possible, it is expected that depth distribution of this biotope will shrink substantially in response to a 50, 70 or 107 cm sea-level rise, without the possibility of recovery, due to the increased depth, leading to a reduction in light availability for photosynthesis. 

There is likely to be considerable variation between sites, the relative contribution of wave surge and exposure to habitat suitability, and the depth range occupied by the biotope. Hence, it is difficult to assess the effect of the different sea-level rise scenarios. However, as the biotope can occur from 0-10 m in depth, it is assumed at a sea-level rise of 50 cm, or 70 cm (middle to high emission scenarios) would have limited effect but that a 107 cm rise (the extreme emission scenario) might result in loss of some of the deeper extent of the biotope in some sites. Therefore, resistance is assessed as ‘High’ under the middle and high emission scenarios so that resilience is ‘High’ and sensitivity assessed as ‘Not sensitive’. But resistance may be ‘Medium’ under the extreme emission scenario so that resilience is ‘Very low’ and sensitivity assessed as ‘Medium’, albeit with ‘Low’ confidence.

Temperature increase (local)

Benchmark. A 5°C increase in temperature for one month, or 2°C for one year. Further detail

Evidence

Saccharina latissima has a latitudinal range of 41.3 degrees South to 79.8 degrees North, depth limits of 2.5 to 30 m, a thermal limit of -1.8 to 21.5°C, and there is little coastline left for poleward range expansion in the northwest Atlantic (Khan et al., 2018). The temperature isotherm of 19 to 20°C has been reported as limiting Saccharina latissima geographic distribution (Müller et al., 2009). The southernmost limit of this species in Europe is northern Portugal (Kerrison et al., 2015; Azevedo et al., 2016), and it grows well between 5 and 17°C (Druehl, 1967, Fortes & Luning, 1980 and Machalek, Davison & Falkowski, 1996 cited in Kerrison et al., 2015). At the high end of this temperature range, net photosynthesis declines and acclimation effort increases, involving the upregulation of many temperature-responsive genes (Davison, 1991 and Heinrich et al., 2012 cited in Kerrison et al., 2015). Tissue loss or death is commonly reported for this species above 17 to 20°C (Gerard & Du Bois, 1988; Gerard, Dubois & Greene, 1987 cited in Kerrison et al., 2015).

Temperature ecotypes may exist which have adapted to high seasonal temperature exposure. For example, populations from Helgoland, Germany, can tolerate temperatures of 18 to 20°C (Davison, 1987 cited in Kerrison et al., 2015), while populations in New York, USA, can survive at >20°C, albeit with substantially reduced growth (Gerard & Du Bois, 1988). In addition, Azevedo et al. (2016) cultured Saccharina latissima in tanks in northwest Portugal throughout the summer, withstanding average temperatures around 20°C from May onwards (temperature varied between 11.7°C in April and 24.9°C in August), well above published optimum temperatures for this species (10 to 15°C). Biomass increased until the third week of May, and afterwards it remained constant until the beginning of July, reaching a density of 13 kg/m ³ (Azevedo et al., 2016). This observation may be explained by their origin in populations located near the southern distribution boundary, which may have acquired adaptations that increased tolerance to high temperatures (Azevedo et al., 2016). In addition, colder water populations of Saccharina latissima may benefit from higher temperatures, with individuals during a warming experiment from Kongsfjorden, Svalbard, having a significantly higher growth rate in the warmer treatments (9°C) compared to colder treatments (4°C) (Iñiguez et al., 2016b).

Elevated temperatures can increase erosion of Saccharina latissima blades and the subsequent release of total organic carbon and total nitrogen. Ding, Brussaard & Timmermans (2025) collected Saccharina latissima samples from the coastal waters south of Texel, The Netherlands, and subjected samples to naturally increased temperatures (from 16.1°C to 22.5°C) and further elevated temperatures (from 16.1°C to 27.1°C). A significant increase in the erosion rate of the distal parts of blades was observed in both temperature treatments, and substantial amounts (4.24 ± 0.31 mg/cm of carbon and 0.32 ± 0.13 mg/cm of nitrogen) of nutrients were released from Saccharina latissima, especially under sub-lethal temperature conditions. Under further elevated temperatures, with a prolonged period of higher temperature and a maximum temperature of 27.1°C, the effects were stronger, and erosion occurred along the edges of the whole blade. Ding, Brussaard & Timmermans (2025) concluded that rising temperatures accelerate the erosion of Saccharina latissima blades, highlighting a reason for the decline of kelp forests under climate change, as well as the potential impacts on nutrient cycling in the oceans.

The growth and uptake of nitrate (NO₃) and phosphate (PO₄³) of juvenile Saccharina latissima sporophytes vary with temperature. Ding, Soetaert & Timmermans (2025) examined this effect under five temperature treatments ranging from 7.6°C to 24.5°C and found that NO₃ uptake significantly decreased when temperature was at or above 15.7°C, while high temperatures had no effect on PO₄³ uptake rates, and nitrate uptake significantly correlated with growth only at lower temperatures of 7.6°C and 12.6°C. In contrast, PO₄³ uptake was significantly correlated with growth across all temperature treatments except the highest (24.5°C). Also, at high temperatures (20.9°C and 24.5°C), NO₃ release was observed, while PO₄³ uptake consistently showed positive values, suggesting distinct regulatory mechanisms for nitrogen and phosphorus in Saccharina latissima (Ding, Soetaert & Timmermans, 2025).

Jung et al. (2025) studied the effects of temperature on early sporophyte development of Saccharina latissima under different temperatures (5, 10, 15, and 20°C) for 20 days. The development of sporophytes was observed earlier at 10°C than all other temperatures, with no sporophytes observed at 20°C during the experiment. Ebbing et al. (2021) also observed optimal reproduction of Saccharina latissima at lower temperatures (10.2°C), but at high light intensities (≥29 µmol photons/m²/s), and at higher temperatures (≥12.6°C) at lower light intensities (≤15 µmol photons/m²/s); highlighting both spring and autumn as the optimal seasons for Saccharina latissima reproduction.

Gametophytes can develop in ≤23°C (Lüning, 1990). However, the optimal temperature range for sporophyte growth is 10 to 15 °C (Bolton & Lüning, 1982). Bolton & Lüning (1982) experimentally observed that sporophyte growth was inhibited by 50 to 70% at 20°C, and following seven days at 23°C, all specimens completely disintegrated. In the field, Saccharina latissima has shown significant regional variation in its acclimation to temperature changes. For example, Gerard & Dubois (1988) observed sporophytes of Saccharina latissima which were regularly exposed to ≥20°C could tolerate these temperatures, whereas sporophytes from other populations, which rarely experience ≥17°C, showed 100% mortality after 3 weeks of exposure to 20°C. Therefore, the response of Saccharina latissima to a change in temperature is likely to be locally variable.

In experiments, Lüning (1980) observed that Chorda filum could not reproduce at 15 to 20°C but found that sporophytes could tolerate ≤26°C.

In the UK, northern to southern Sea Surface Temperature ranges from 8 to 16°C in summer and 6 to 13°C in winter (Beszczynska-Möller & Dye, 2013). The effect of temperature change e is likely to be regionally variable.

Sensitivity assessment. Ecotypes of Saccharina lattisma have been shown to have different temperature optimums (Dubois, 1988; Kerrison et al., 2015; Azevedo et al., 2016). Both a 2 & 5°C increase in temperature, when combined with high UK summer temperatures in the south of the UK, could cause large-scale mortality of Saccharina lattisma and inhibit Chorda filum reproduction. Therefore, resistance has been assessed as ‘None’. Hence, resilience is assessed as ‘High’ and sensitivity as ‘Medium’.

Temperature decrease (local)

Benchmark. A 5°C decrease in temperature for one month, or 2°C for one year. Further detail

Evidence

Saccharina lattissima is widespread throughout the Arctic. The species has a latitudinal range of 41.3 degrees South to 79.8 degrees North, depth limits of 2.5 to 30 m, a thermal limit of -1.8 to 21.5°C, and there is little coastline left for poleward range expansion in the northwest Atlantic (Khan et al., 2018). Saccharina latissima is known to grow well between 5 and 17°C (Druehl, 1967, Fortes & Luning, 1980 and Machalek, Davison & Falkowski, 1996 cited in Kerrison et al., 2015), and it has a lower temperature threshold for sporophyte growth at 0°C (Lüning, 1990).

Despite a low temperature tolerance, growth at low temperatures does affect the demography of natural kelp beds through a reduction in growth rate combined with increased longevity (Rinde & Sjøtun, 2005). Novaczek et al. (1986) observed that 99% of newly settled zoospores died at 0°C, but sporophytes transferred from 5°C to 0°C remained healthy and continued to grow for a period of two months. Novaczek et al. (1986) therefore demonstrated that sporophytes could tolerate exposure to low (≥0°C) temperatures, but that exposure could have negative effects on larval survival and recruitment processes. However, a reduction in growth due to cold temperatures may just be representative of that specific population of kelp. For example, Saccharina latissima grows in Danish waters below the low optimal temperature for sporophyte growth (10 to 15°C), but those temperatures reflect the natural autumn and winter temperatures of the region (Boderskov et al., 2016).

Jung et al. (2025) studied the effects of temperature on early sporophyte development of Saccharina latissima under different temperatures (5, 10, 15, and 20°C) for 20 days, and the development of sporophytes was observed earlier at 10°C than all other temperatures, with no sporophytes observed at 20°C during the experiment. Ebbing et al. (2021) also observed optimal reproduction of Saccharina latissima at lower temperatures (10.2°C), but at high light intensities (≥29 µmol photons/m²/s), and at higher temperatures (≥12.6°C) at lower light intensities (≤15 µmol photons/m²/s); highlighting both spring and autumn as the optimal seasons for Saccharina latissima reproduction.

The growth and uptake of nitrate (NO₃) and phosphate (PO₄³) of juvenile Saccharina latissima sporophytes vary with temperature. Ding, Soetaert & Timmermans (2025) examined this effect under five temperature treatments ranging from 7.6°C to 24.5°C and found that NO₃ uptake significantly decreased when temperature was at or above 15.7°C, while high temperatures had no effect on PO₄³ uptake rates, and nitrate uptake significantly correlated with growth only at lower temperatures of 7.6°C and 12.6°C. In contrast, PO₄³ uptake was significantly correlated with growth across all temperature treatments except the highest (24.5°C). Also, at high temperatures (20.9°C and 24.5°C), NO₃ release was observed, while PO₄³ uptake consistently showed positive values, suggesting distinct regulatory mechanisms for nitrogen and phosphorus in Saccharina latissima (Ding, Soetaert & Timmermans, 2025).

Monteiro et al. (2021) studied the acclimation mechanisms of Saccharina latissima towards temperature and salinity. Samples of Saccharina latissima sporophytes were collected from Brittany, France, and were exposed to a combination of three temperatures (0, 8 and 15°C) and two salinity levels (20 and 30 psu). The Saccharina latissima samples experienced a fivefold increase in the osmolyte mannitol in response to low temperature (0°C) compared to 8 and 15°C, which may have ecological and economic implications (Monteiro et al., 2021). Low temperatures significantly affected all parameters, mostly in a negative way; chlorophyll-a, the accessory pigment pool, growth and the maximal quantum yield of photosystem II (Fv/Fm) were significantly lower at 0°C, while the de-epoxidation state (the light-harvesting state, aka how plants dissipate excess light energy as heat) of the xanthophyll cycle (a mechanism protecting plants against oxidative stress) was increased at both 0 and 8°C compared to 15°C (Monteiro et al., 2021).

Chorda filum is also widespread throughout the Arctic, and its sporophytes can tolerate 0°C (Novaczek et al., 1986). Subtidal red algae can survive at -2°C (Lüning, 1990; Kain & Norton, 1990). The distribution and temperature tolerances of these species suggest they are likely to be unaffected by temperature decreases assessed within this pressure.

In the UK, the northern to southern Sea Surface Temperature ranges from 8 to 16°C in summer and 6 to 13°C in winter (Beszczynska-Möller & Dye, 2013). The effect of temperature change is likely to be regionally variable.

Sensitivity assessment. Resistance has been assessed as ‘High’, resilience as ‘High’. Sensitivity has been assessed as ‘Not Sensitive’.

Salinity increase (local)

Benchmark. A increase in one MNCR salinity category above the usual range of the biotope or habitat. Further detail

Evidence

For macroalgae, a salinity of 33 to 35 psu commonly results in optimal growth, while areas of reduced salinity, such as at river mouths, are amenable to the survival of fewer, tolerant species (Kerrison et al., 2015). Saccharina latissima can be classed as semi-euryhaline but appears more sensitive to salinity than other kelp, such as Laminaria digitata (Kerrison et al., 2015). No reduction in growth rates is observed between 24 and 35 psu, and there may even be an increase (Druehl, 1967 and Gerard, Dubois & Greene, 1987 cited in Kerrison et al., 2015). Between 25 and 55 psu (under acute 2- and 5-day exposure), Saccharina latissima shows a high photosynthetic ability at >80% (Karsten, 2007). Below 24 psu, a stress response can be observed: a 20 to 25% reduction in growth rate at 21 psu (Dubois & Greene, 1987 cited in Kerrison et al., 2015) and a 20 to 30% reduction in photosynthetic performance at 15 to 20 psu (Karsten, 2007). After two days at 5 psu, Saccharina latissima showed a significant decline in photosynthetic ability at approx. 30% of control, and after five days at 5 psu, Saccharina latissima specimens became bleached and showed signs of severe damage (Karsten, 2007). The experiment by Karsten (2007) was conducted on Saccharina latissima from the Arctic, and they suggest that acclimation to rapid salinity changes could be slower at extremely low water temperatures (1 to 5°C) than at temperate latitudes. It is therefore possible that the resident Saccharina latissima of the UK may be able to acclimate to salinity changes more effectively. However, Birkett et al. (1998b) suggested that kelps are stenohaline and therefore long-term increases in salinity may be detrimental.

Natural populations do occur at salinity tipping points, such as those in the White Sea, where salinity is 24 to 26 psu (Drobyshev, 1971 cited in Kerrison et al., 2015) and in Danish fjords, where salinity is 22 to 24 psu (Middelboe and Sand-Jensen, 2000 cited in Kerrison et al., 2015); however, these may represent locally adapted ecotypes. Nielsen et al. (2016) studied two populations of Saccharina latissima in Danish waters, one brackish and one marine, and noted how gene flow was reduced both between clusters and between populations within clusters. Thus, highlighting the high likelihood of locally adapted ecotypes, with both populations vulnerable to differing changes in salinity, such as an increase in salinity in the brackish ecotype, or a decrease in salinity in the marine ecotype (Nielsen et al., 2016).

Chorda filum can be found in rock pools (South & Burrows, 1967). High air temperatures cause surface evaporation of water from rock pools, so that salinity steadily increases. The extent of temperature and salinity change is affected by the frequency and time of day at which tidal inundation occurs. If high tide occurs in early morning and evening, the diurnal temperature follows that of the air, whilst high water at midday suddenly returns the temperature to that of the sea (Pyefinch, 1943). It should be noted, however, that local populations may be acclimated to the prevailing salinity regime and may therefore exhibit different tolerances than other populations subject to different salinity conditions, and therefore, caution should be used when inferring tolerances. However, it is likely that Chorda filum is tolerant of short-term salinity increases.

Sensitivity assessment. The evidence suggests that Saccharina latissima and Chorda filum can tolerate short-term exposure to hypersaline conditions (≥40‰-MNCR full salinity). An increase in salinity to ≥40‰ may, however, be above the optima for characterizing species and cause a decline in growth, and possibly loss of red algae and a reduction in species diversity. Resistance has been assessed as ‘Medium’, resilience as ‘High’. The sensitivity of this biotope to an increase in salinity has been assessed as ‘Low’.

Salinity decrease (local)

Benchmark. A decrease in one MNCR salinity category above the usual range of the biotope or habitat. Further detail

Evidence

For macroalgae, a salinity of 33 to 35 psu commonly results in optimal growth, while areas of reduced salinity, such as at river mouths, are amenable to the survival of fewer, tolerant species (Kerrison et al., 2015). Saccharina latissima can be classed as semi-euryhaline but appears more sensitive to salinity than other kelp, such as Laminaria digitata (Kerrison et al., 2015). No reduction in growth rates is observed between 24 and 35 psu, and there may even be an increase (Druehl, 1967 and Gerard, Dubois & Greene, 1987 cited in Kerrison et al., 2015). Between 25 and 55 psu (under acute 2- and 5-day exposure), Saccharina latissima shows a high photosynthetic ability at >80% (Karsten, 2007). Below 24 psu, a stress response can be observed: a 20 to 25% reduction in growth rate at 21 psu (Dubois & Greene, 1987 cited in Kerrison et al., 2015) and a 20 to 30% reduction in photosynthetic performance at 15 to 20 psu (Karsten, 2007). After two days at 5 psu, Saccharina latissima showed a significant decline in photosynthetic ability at approx. 30% of control, and after five days at 5 psu, Saccharina latissima specimens became bleached and showed signs of severe damage (Karsten, 2007). The experiment by Karsten (2007) was conducted on Saccharina latissima from the Arctic, and they suggest that at extremely low water temperatures (1 to 5°C) macroalgae acclimation to rapid salinity changes could be slower than at temperate latitudes. It is therefore possible that the resident Saccharina latissima of the UK may be able to acclimate to salinity changes more effectively.

Natural populations do occur at salinity tipping points, such as those in the White Sea, where salinity is 24 to 26 psu (Drobyshev, 1971 cited in Kerrison et al., 2015) and in Danish fjords, where salinity is 22 to 24 psu (Middelboe and Sand-Jensen, 2000 cited in Kerrison et al., 2015). However, these may represent locally adapted ecotypes. Nielsen et al. (2016) studied two populations of Saccharina latissima in Danish waters, one brackish and one marine, and noted how gene flow was reduced both between clusters and between populations within clusters. Thus, highlighting the high likelihood of locally adapted ecotypes, with both populations vulnerable to differing changes in salinity, such as an increase in salinity in the brackish ecotype, or a decrease in salinity in the marine ecotype (Nielsen et al., 2016).

Young Saccharina latissima sporophytes can survive a four-day exposure to 11 psu, although significant stress is observed (Peteiro & Sánchez, 2012 cited in Kerrison et al., 2015), while exposure of only a few days to 5 or 6 psu results in either a 95% reduction in photosynthetic performance and significant pigment loss, or death (Karsten, 2007; Peteiro & Sánchez, 2012 cited in Kerrison et al., 2015). Monteiro et al. (2021) studied the acclimation mechanisms of Saccharina latissima towards temperature and salinity. Samples of Saccharina latissima sporophytes were collected from Brittany, France, and were exposed to a combination of three temperatures (0, 8 and 15°C) and two salinity levels (20 and 30 psu). Mannitol content and growth decreased with decreasing salinity; in contrast, pigment content and maximal quantum yield of photosystem II were to a large extent unresponsive to salinity (Monteiro et al., 2021).

Vettori, Nikora & Biggs (2020) studied the implications of hyposaline (freshwater) stress on the morphological and mechanical properties of Saccharina latissima. They noted how under hyposaline stress blades bleach, develop blisters underneath the cortex, change dimensions (increased volume and thickness, decreased width), and how blade material becomes more flexible and more difficult to break (i.e. tougher). However, it is important to note that the response to hyposaline stress reported may be specific to seaweeds living in waters with high salinity (salinity at the sample collection site is around 30‰ and samples were held in tanks of 34‰) (Vettori, Nikora & Biggs, 2020).

Chorda filum is tolerant of low salinities (Wilce, 1959; Hayren, I940; Norton & South, 1969), and has been recorded at Björnholm, Finland, at a salinity as low as 5.15‰ (Hayren, 1940). Norton & South (1969) observed that Chorda filum could develop sporophytes at ≥5‰ under laboratory conditions, however, at low salinities, the time taken to develop into sporophytes took 65 days at 5‰, or 16 days at 35‰. It was also noted that below 9‰, sporophytes did not grow above 2 mm in length.

Sensitivity assessment. A decrease in one MNCR salinity scale from “Full” (30 to 40 psu) to “Reduced” (18 to 30 psu) could inhibit Saccharina lattissima photosynthesis and hence growth. Chorda filum is highly tolerant of low salinity and is unlikely to be affected at the benchmark level. However, a shift to reduced salinity conditions is likely to result in a change in the infauna community and an overall reduction in species diversity. Therefore, resistance has been assessed as ‘Medium’ and resilience as ‘High’. Sensitivity of this biotope to a decrease in salinity has been assessed as ‘Low’.

Water flow (tidal current) changes (local)

Benchmark. A change in peak mean spring bed flow velocity of between 0.1 m/s to 0.2 m/s for more than one year. Further detail

Evidence

The presence of water motion is important for kelp. The diffusive boundary layer becomes thinner as water motion increases, and is further reduced by thallus pitching and flapping (Denny & Roberson, 2002 and Huang, Rominger & Nepf cited in Kerrison et al., 2015). The rates of photosynthesis and nutrient uptake increase in correlation to the current velocity until these metabolic processes become saturated (Wheeler, 1980, Hurd, 2000 and Hepburn et al., 2007 cited in Kerrison et al., 2015). This occurs with current velocities of approximately 0.1 m/s, with anything less defined as low water motion (Wheeler, 1980 cited in Kerrison et al., 2015). Anything above 0.25 m/s can be considered as high water motion (Stevens & Hurd, 1997 cited in Kerrison et al., 2015).

Fast flow may also reduce the settlement and abundance of epiphytes, grazers, and sediment, which could smother or degrade the macroalgae, reducing growth rate (Méléder et al., 2010 cited in Kerrison et al., 2015). Nevertheless, moderate wave exposure may lead to increased invertebrate abundance (Norderhaug et al., 2012 cited in Kerrison et al., 2015). Very high-water motion may, however, be counterproductive, increasing the dislodgement of fully-grown adults (Hurd, 2000 cited in Kerrison et al., 2015). On exposed coasts, kelps can be exposed to wind-wave induced orbital water velocities as high as 2 to 3 m/s (de Bettignies, Wernberg & Lavery, 2013 cited in Kerrison et al., 2015). Macroalgae grown in such high-water motion invest more energy into the development of large holdfasts to attach more firmly to the substratum (Kawamata, 2001, Duggins et al., 2003 cited in Kerrison et al., 2015), at the cost of reduced growth size and productivity (Gerard & Mann, 1979 cited in Kerrison et al., 2015).

Flow rate also has a considerable effect on the blade and stipe morphology of kelps. In sheltered conditions, the thallus becomes wide and thin, and in some species, corrugated, such as Saccharina latissima (Kerrison et al., 2015). It is thought that the increased surface area enables maximal photon capture and gas/nutrient exchange, while thallus undulations may increase turbulence (Hurd, 2000, Fowler-Walker, Wernberg & Connell, 2006, Wing et al., 2007, Koehl, 2008, Hurd & Pilditch, 2009 cited in Kerrison et al., 2015). In exposed conditions, the thallus becomes narrow and thick with a robust stipe (Hurd, 2000 cited in Kerrison et al., 2015). This makes the thallus more hydrodynamic and reduces its drag, which is reasoned to prevent breakage or dislodgement of the adult sporophyte (Fowler-Walker, Wernberg & Connell, 2006 and Hurd & Pilditch, 2009 cited in Kerrison et al., 2015).

Saccharina latissima prefers low to moderate water motion areas and is usually absent in locations with high motion or surf (Southward & Orton, 1954, Druehl, 1967 and Burrows, 2012 and cited in Kerrison et al., 2015). However, Saccharina latissima has been successfully cultured in high motion areas with currents up to 1.53 m/s (Buck & Buchholz, 2005 cited in Kerrison et al., 2015). This suggests that their exclusion from high water motion locations may be due to other factors, such as competition for space by a species more adapted to this environment.

Mols-Mortensen et al. (2017) cultivated Saccharina latissima with different wave and current exposures (sheltered, current-exposed and wave-exposed) in the Faroe Islands (from March to August 2015) to understand their variation in growth, yield, and protein concentration. Location 1 was defined as the sheltered location with a current speed of <5 cm/s (0.05 m/s) approx. half of the time and an overall current speed of <10 cm/s (0.1 m/s). The maximum observed current speed on this location was 20 to 30 cm/s (0.2 to 0.3 m/s), but this was only observed for a short period of time (Mortensen et al. 2014b cited in Mols-Mortensen et al., 2017). The highest average wave heights on location 1 were 0.9 m, and therefore, the location was considered to be sheltered, both with regard to current speed and wave heights. Location 2 was defined as the current-exposed location with an overall current speed of >20 cm/s  (>0.2 m/s) and occasional current speeds of >40 cm/s (>0.4 m/s) not maintained for long periods of time (Larsen 1999 cited in Mols-Mortensen et al., 2017). The highest average wave heights on location 2 were 0.9 m, and therefore, current speed was considered to be the most important exposure factor on this location. Location 3 was defined as the wave-exposed location with a current speed of <10 cm/s (<0.1 m/s) approx. half of the time and an overall current speed of <20 cm/s (<0.2 m/s). Single observations on current speeds of 40 to60 cm/s (0.4 to 0.6 m/s) were reported (Mortensen et al. 2014a cited in Mols-Mortensen et al., 2017). The highest average wave heights on location 3 were 2.2 m, and therefore, wave height was considered to be the most important exposure factor on this location. Overall, Saccharina latissima individuals cultivated at the current exposed location were heavier compared to the individuals cultivated at the other locations; however, the total biomass yield was significantly lower at the current exposed location (Mols-Mortensen et al., 2017).

Peteiro & Freire (2013) measured Saccharina latissima growth from two sites; the first had maximal water velocities of 0.3 m/sec, and the second 0.1 m/sec. At site one, Saccharina latissima had significantly larger biomass than at site two (16 kg/m to 12 kg/m, respectively). Peteiro & Freire (2013) suggested that faster water velocities were beneficial to Saccharina latissima growth. However, Gerard & Mann (1979) measured Saccharina latissima productivity at greater water velocities and found that Saccharina latissima productivity was reduced in moderately strong tidal streams (≤1 m/sec) when compared to weak tidal streams (<0.5 m/sec).

Chorda filum sporophytes often grow on unstable objects, such as pebbles and shells. Owing to the typically unstable substratum on which Chorda filum grows, whole populations can be moved during storms and deposited in more sheltered locations where development will continue (South & Burrows, 1967). The survival of Chorda filum sporophytes following transport of their attached substrata indicates the species is relatively tolerant to changes in water flow or wave action.

Sensitivity assessment. SS.SMp.KSwSS.SlatR and its sub-biotopes are recorded from sites in strong to very weak tidal flow and from very wave-exposed to extremely wave-sheltered conditions. Sub-biotopes vary across this spectrum, with lower water movement in sandy and muddy examples of the biotope, and mixed gravels and coarse sediments (cobbles and pebbles) in stronger water movement, especially due to wave action (Connor et al., 2004). Hence, the abundance of Saccharina latissima varies between sub-biotopes together with the nature of the associated macroalgal and infaunal community.

As the degree of wave and/or tidal exposure decreases, there is a change in community structure, with the density of Laminaria and the diversity of red algal species increasing (SS.SMp.KSwSS.SlatR.Gv) (Connor et al., 2004; JNCC, 2015, 2022). A change in tidal flow of 0.1 to 0.2 m/sec in low energy biotopes, e.g. SS.SMp.KSwSS.SlatR.Mu, may, however, remove finer sediment fractions (e.g. mud) and may therefore change the biotope. However, it may not adversely affect the Saccharina latissima component as long as the hard substrata used for attachment remains, although algal mats of Bonnemaisonia hamifera may be (see SS.SMp.KSwSS.SlatR.Mu). However, evidence is lacking, and a change in tidal velocities at the pressure benchmark is not likely to result in a significant change to the dominant species.

Hence, resistance has been assessed as ‘High’, resilience as ‘High’. Sensitivity has been assessed as ‘Not Sensitive’ at the benchmark level.

Emergence regime changes

Benchmark.  1) A change in the time covered or not covered by the sea for a period of ≥1 year or 2) an increase in relative sea level or decrease in high water level for ≥1 year. Further detail

Evidence

SS.SMp.KSwSS.SlatR and SS.SMp.KSwSS.SlatCho are recorded from 0-10m, while SlatR can extend to 20m (Connor et al., 2004). Therefore the upper limit of the biotopes in the sub-littoral fringe (South & Burrows, 1967; White & Marshall, 2007) could be exposed during some low tides.

An increase in emergence will result in an increased risk of desiccation and mortality of Saccharina latissima and Chorda filum. Removal of macroalgae canopy may also increase desiccation and mortality of the undergrowth red seaweed community (Hawkins & Harkin, 1985). Providing that suitable substrata are present, the biotope is likely to re-establish further down the shore within a similar emergence regime to that which existed previously.

Sensitivity assessment. Resistance has been assessed as ‘Medium’. Resilience as ‘High’. The sensitivity of this biotope to a change in emergence is considered as ‘Low’.

Wave exposure changes (local)

Benchmark. A change in near shore significant wave height of >3% but <5% for more than one year. Further detail

Evidence

Saccharina latissima typically dominates sheltered shorelines (Gilson et al., 2023) and is rarely present in areas of wave exposure, where it is out-competed by Laminaria hyperborea (Birkett et al., 1998). However, off the coast of northern Portugal, Saccharina latissima grew in offshore exposed conditions, with growth rates of 3.3% to 4.5%/day between January and May, while withstanding high wave heights (ranging from 0.5 to 12.6 m during the study period of January to September) (Azevedo et al., 2019). Chorda filum sporophytes often grow on unstable objects, such as pebbles and shells. Owing to the typically unstable substratum on which Chorda filum grows, whole populations can be moved during storms and deposited in more sheltered locations where development will continue (South & Burrows, 1967).

Mols-Mortensen et al. (2017) cultivated Saccharina latissima with different wave and current exposures (sheltered, current-exposed and wave-exposed) in the Faroe Islands (from March to August 2015) to understand their variation in growth, yield, and protein concentration. Location 1 was defined as the sheltered location with a current speed of <5 cm/s (0.05 m/s) approx. half of the time and an overall current speed of <10 cm/s (0.1 m/s). The maximum observed current speed on this location was 20 to 30 cm/s (0.2 to 0.3 m/s), but this was only observed for a short period of time (Mortensen et al. 2014b cited in Mols-Mortensen et al., 2017). The highest average wave heights on location 1 were 0.9 m, and therefore, the location was considered to be sheltered, both with regard to current speed and wave heights. Location 2 was defined as the current-exposed location with an overall current speed of >20 cm/s (0.2 m/s) and occasional current speeds of >40 cm/s (0.4 m/s) not maintained for long periods of time (Larsen 1999 cited in Mols-Mortensen et al., 2017). The highest average wave heights on location 2 were 0.9 m, and therefore, current speed was considered to be the most important exposure factor on this location. Location 3 was defined as the wave-exposed location with a current speed of <10 cm/s (0.1 m/s) approx. half of the time and an overall current speed of <20 cm/s (0.2 m/s). Single observations on current speeds of 40 to 60 cm/s (0.4 to 0.6 m/s) were reported (Mortensen et al. 2014a cited in Mols-Mortensen et al., 2017). The highest average wave heights on location 3 were 2.2 m, and therefore, wave height was considered to be the most important exposure factor on this location. Overall, Saccharina latissima individuals cultivated at the current exposed location were heavier compared to the individuals cultivated at the other locations; however, the total biomass yield was significantly lower at the current exposed location (Mols-Mortensen et al., 2017).

Zhu et al. (2021) studied the morphological and physiological plasticity of Saccharina latissima in response to different hydrodynamic conditions and nutrient availability (56 days under fully controlled conditions of waves or no waves, and high or low nutrients). They observed how waves primarily increased frond biomass, elongation rate, and carbon to nitrogen ratio (C:N ratio), and induced both a greater variety in and rougher frond surface shapes; the highest C:N ratio was observed in the low nutrient-wave treatment. Together, these results seem to suggest that the thready and spring-like shapes found in the central frond (i.e., rougher frond surface) in wave-exposed conditions can at least partly compensate for low nutrient availability by enhancing nutrient and photon acquisition, particularly in low nutrient conditions (Zhu et al., 2021). Zhu et al. (2021) concluded that frond surface shapes in the newly formed central frond of Saccharina latissima can be regarded as possessing high morphological and physiological plasticity that enables kelp to cope with contrasting environments.

Visch, Nylund & Pavia (2020) studied the effect of wave exposure (defined as 500,000 to 800,000 m²/s for exposed, 100,000 to 200,000 m²/s for moderately exposed, and 10,000 to 30,000 m²/s for sheltered) on growth and biofouling of Saccharina latissima along the Swedish west coast. Growth, measured as blade surface area, generally increased with decreased wave exposure, with approximately 40% less growth at exposed locations compared to sheltered or moderately exposed locations (Visch, Nylund & Pavia, 2020). Biofouling of kelp decreased with increased wave exposure, from 10 and 6% coverage at sheltered and moderately exposed locations, respectively, to 3% at exposed locations (Visch, Nylund & Pavia, 2020). In addition, exposure level affected the tissue composition, with a high carbon, but low nitrogen and water content at exposed locations compared to moderate and sheltered sites; isotope signatures (i.e. δ¹³C and δ¹⁵N) also differed between exposure levels (Visch, Nylund & Pavia, 2020).

Gilson et al. (2023) studied the seasonal and spatial variability in rates of primary production and detritus release by intertidal stands of Saccharina latissima on wave-exposed shores in the northeast Atlantic. On moderately exposed shores, productivity and erosion of Saccharina latissima remained low and showed no clear seasonal pattern (Gilson et al., 2023). Peak erosion rates of Saccharina latissima at both wave exposures were approx. 0.6 g dry weight/day (Gilson et al., 2023), which is higher than previous rates recorded for populations of other kelp species, such as L. hyperborea and L. ochroleuca, along the UK coastline (Pessarrodona, Moore, et al., 2018 cited in Gilson et al., 2023). The ruffled margins of Saccharina latissima create considerably more drag than the flat lamina of Laminaria digitata, accounting for their greater rates of dislodgment even at more sheltered sites (Buck & Buchholz, 2005 cited in Gilson et al., 2023). In addition, Saccharina latissima also routinely settles on semi-stable rocks and cobbles instead of emergent bedrock, particularly in sheltered conditions, increasing its susceptibility to dislodgement (Scheibling et al., 2009 and Smale & Vance, 2016 cited in Gilson et al., 2023).

Storm-induced increases in wave action can be detrimental to kelp biotopes. During the North East Atlantic storm season of 2013 to 2014, the south coast of the UK was subjected to some of the most intense storms in recent history, being classed as a '1-in-30 year' event, where inshore significant wave heights and periods exceeded 7 m and 13 seconds (Smale & Vance, 2015). Overall, kelp canopies were highly resistant to storm disturbance, however, at one study site, a mixed canopy comprising Laminaria ochroleuca, Saccharina latissima, and Laminaria hyperborea was significantly altered by the storms, due to a decreased abundance of the former two species (Smale & Vance, 2015). On the Atlantic coast of Nova Scotia, Hurricane Earl generated extreme wave heights of up to 25 m and strong bottom currents, which caused a large-scale defoliation of kelp beds in shallow subtidal zones (Filbee-Dexter & Scheibling, 2012). Saccharina latissima and Laminaria digitata were stripped of blades, leaving only stipes and fragments, resulting in a 46% average loss of kelp canopy cover across the surveyed sites; the strong bottom currents also caused the displacement of urchins (Strongylocentrotus droebachiensis) (Filbee-Dexter & Scheibling, 2012). In addition, coralline and filamentous red algae cover increased after the storm due to the loss of kelp (Filbee-Dexter & Scheibling, 2012).

Sensitivity assessment. SS.SMp.KSwSS.SlatR and its sub-biotopes are recorded from sites in strong to very weak tidal flow and from very wave-exposed to extremely wave-sheltered conditions. Sub-biotopes vary across this spectrum, with lower water movement in sandy and muddy examples of the biotope, and mixed gravels and coarse sediments (cobbles and pebbles) in stronger water movement, especially due to wave action (Connor et al., 2004). Hence, the abundance of Saccharina latissima varies between sub-biotopes, together with the nature of the associated macroalgal and infaunal community. Saccharina latissima has been reported to grow in high wave exposure environments (Mols-Mortensen et al., 2017; Azevedo et al., 2019; Visch, Nylund & Pavia, 2020).

Connor et al. (2004) noted that sub-biotopes within SS.SMp.KSwSS.SlatR are largely distinguished by wave exposure. A large increase in near-shore wave height is likely to significantly influence biotope structure, especially where wave action mobilises the sediment and removes hard substratum embedded in the sediment, on which the macroalgae depend for attachment, potentially increase dislodgment or relocation of the characterizing species (South & Burrows, 1967; Birkett et al., 1998b; Smale & Vance, 2015), or increase erosion (Gilson et al., 2023) and affect growth (Mols-Mortensen et al., 2017). Storm damage (as above) can even remove kelps from hard substrata.

However, an increase in nearshore significant wave height of 3 to 5% is not likely to result in damage to or loss of this biotope, but may result in subtle changes to the associated community depending on the interplay of wave action and tidal streams. For example, sub-biotopes that occur predominantly in very wave-sheltered conditions may transition into sub-biotopes typical of sheltered conditions but not lost. Therefore, resistance has been assessed as ‘High’, resilience as ‘High’, and sensitivity as ‘Not Sensitive’ at the benchmark level.

Transition elements & organo-metal contamination

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

This pressure is Not assessed but evidence is presented where available

Bryan (1984) suggested that the general order for heavy metal toxicity in seaweeds is: Organic Hg > inorganic Hg > Cu > Ag > Zn > Cd > Pb. Cole et al., (1999) reported that Hg was very toxic to macrophytes. Similarly, Hopkin & Kain (1978) demonstrated sub-lethal effects of heavy metals on kelp gametophytes and sporophytes, including reduced growth and respiration. Sheppard et al. (1980) noted that increasing levels of heavy metal contamination along the west coast of Britain reduced species number and richness in holdfast fauna, except for suspension feeders which became increasingly dominant. Gastropods may be relatively tolerant of heavy metal pollution (Bryan, 1984). Although macroalgae species may not be killed, except by high levels of contamination, reduced growth rates may impair the ability of the biotope to recover from other environmental disturbances. Thompson & Burrows (1984) observed the growth of Saccharina latissima sporophyte growth was significantly inhibited at 50 µg Cu /l, 1000 µg Zn/l and 50 µg Hg/l. Zoospores were found to be more intolerant and significant reductions in survival rates were observed at 25 µg Cu/l, 1000 µg Zn/l and 5 µg/l.

Hydrocarbon & PAH contamination

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

This pressure is Not assessed but evidence is presented where available

The mucilaginous slime layer coating of Laminarians may protect them from smothering by oil. Hydrocarbons in solution reduce photosynthesis and may be algicidal. However, Holt et al. (1995) reported that oil spills in the USA and from the 'Torrey Canyon' had little effect on kelps. Similarly, surveys of subtidal communities at a number sites between 1-22.5m below chart datum showed no noticeable impacts of the Sea Empress oil spill and clean up (Rostron & Bunker, 1997) or during experimental release of untreated oil in Baffin Island, Canada (Cross et al., 1987). Laboratory studies of the effects of oil and dispersants on several red algae species (Grandy 1984) concluded that they were all sensitive to oil/ dispersant mixtures, with little differences between adults, sporelings, diploid or haploid life stages.

Synthetic compound contamination

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

This pressure is Not assessed but evidence is presented where available

O'Brian & Dixon (1976) suggested that red algae were the most sensitive group of macrophytes to oil and dispersant contamination (see Smith, 1968). Saccharina latissima has also been found to be sensitive to antifouling compounds. Johansson (2009) exposed samples of Saccharina latissima to several antifouing compounds, observing chlorothalonil, DCOIT, dichlofluanid and tolylfluanid inhibited photosynthesis. Exposure to Chlorothalonil and tolylfluanid, was also found to continue inhibiting oxygen evolution after exposure had finished, and may cause irreversible damage.

Smith (1968) observed that epiphytic and benthic red algae were intolerant of dispersant or oil contamination during the Torrey Canyon oil spill; only the epiphytes Crytopleura ramosa and Spermothamnion repens and some tufts of Jania rubens survived together with Osmundea pinnatifida, Gigartina pistillata and Phyllophora crispa from the sublittoral fringe.

Radionuclide contamination

Benchmark. An increase in 10µGy/h above background levels. Further detail

Evidence

No evidence

Introduction of other substances

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

This pressure is Not assessed.

De-oxygenation

Benchmark. Exposure to dissolved oxygen concentration of less than or equal to 2 mg/l for one week (a change from WFD poor status to bad status). Further detail

Evidence

Reduced oxygen concentrations can inhibit both photosynthesis and respiration in macroalgae (Kinne, 1977). Despite this, macroalgae are thought to buffer the environmental conditions of low oxygen, thereby acting as a refuge for organisms in oxygen depleted regions especially if the oxygen depletion is short-term (Frieder et al., 2012). A rapid recovery from a state of low oxygen is expected if the environmental conditions are transient. If levels do drop below 4 mg/l negative effects on these organisms can be expected with adverse effects occurring below 2 mg/l (Cole et al., 1999).

Sensitivity Assessment. Reduced oxygen levels are likely to inhibit photosynthesis and respiration but not cause a loss of the macroalgae population directly. Resistance has been assessed as ‘High’ and resilience as ‘High’. Sensitivity has been assessed as ‘Not sensitive’ at the benchmark level.

Nutrient enrichment

Benchmark. Compliance with WFD criteria for good status. Further detail

Evidence

Areas with high nutrient loading will sustain rapid macroalgal growth during the summer (Davison, Andrews & Stewart, 1984 cited in Kerrison et al., 2015), and where nutrient loading is lower, high water flow increases the nutrient uptake rate of macroalgae by refreshing the boundary layer (Wheeler & North (date) cited in Kerrison et al., 2015), so maximal growth rates can be sustained. It has been shown in Saccharina latissima that 10 μmol/l of nitrate is required to maximise growth rate and leads to internal storage for later use (Chapman, Markham & Lüning, 1978 cited in Kerrison et al., 2015). Boderskov et al. (2016) noted how Saccharina latissima grown under high nutrient availability in Denmark fulfils a higher degree of nutrient bioremediation, and has an improved biomass quality in regard to increased concentrations of pigments and nitrogen-rich compounds.

Conolly & Drew (1985) found Saccharina latissima sporophytes had relatively higher growth rates when in close proximity to a sewage outlet in St Andrews, UK, compared to other sites along the east coast of Scotland. At St Andrews, nitrate levels were 20.22 µM, which represents an approx. 25% increase compared to other sites (approx. 15.87 µM). Read et al. (1983) reported that after the installation of a new sewage treatment works, which reduced the suspended solid content of liquid effluent by 60% in the Firth of Forth, Saccharina latissima became abundant where previously it had been absent.

The association of fish and shellfish mariculture can also lead to an increased growth rate of macroalgae, while removing excess nutrients from the environment (Sanderson et al, 2008, Sanderson et al., 2012 and Wang et al., 2014 cited in Kerrison et al., 2015). Handå et al. (2013) reported Saccharina latissima sporophytes grew approx. 1% faster per day when in close proximity to Norwegian salmon farms, where elevated ammonium could be readily absorbed by sporophytes. However, experimentation in Denmark did not show any benefit in terms of growth, nitrogen, phosphorus, or amino acid content of Saccharina latissima cultured in proximity to fish and mussel aquaculture (Marinho, Holdt & Angelidaki, 2015 and Marinho et al., 2015 cited in Kerrison et al., 2015).

Rugiu et al. (2021) exposed Saccharina latissima to natural seawater, water enriched to levels of ammonium and nitrate simulating finfish cage waste (test IMTA1), and a combination of such enrichment with natural effluents coming from mussels (test IMTA2). The Saccharina latissima biomass was higher and produced elevated total organic content when exposed to both IMTA1 and IMTA2 nutrient scenarios, including a significant enhancement in pigment content only when algae were exposed to the strongest enrichment (IMTA2) (Rugiu et al., 2021). In addition, the photosynthetic responses in terms of relative electron transfer rate, PSII (photosystem II) saturation irradiance, total nitrogen content, and the content of chlorophyll-a and fucoxanthin were also positively affected by both IMTA1 and IMTA2 (Rugiu et al., 2021). Rugiu et al. (2021) concluded that Saccharina latissima showed a significant physiological response to nutrient enrichment mimicking aquaculture settings, as well as the benefit of added nutrients through a boost in photosynthetic activity that leads to higher kelp biomass and pigment production.

Fales et al. (2023) compared the physiological responses of Saccharina latissima sporophytes to high temperature stress (low: 9 and 13°C, moderate: 15 and 16°C, and warm: 21°C) and nitrogen limitation (low: 1 to 3 μM vs. high: >10 μM) over eight to nine days. Saccharina latissima responded negatively to elevated temperatures, but not to low nitrogen levels. Blades of Saccharina latissima showed signs of metabolic stress and reduced growth in the warmest temperature treatment (21°C), at both high and low nitrogen levels, suggesting that Saccharina latissima is susceptible to thermal stress over short time periods, and that nutrient additions may actually reduce kelp performance at supra-optimal temperatures (Fales et al., 2023).

Jevne, Forbord & Olsen (2020) examined how differences in light conditions and nutrient availability affect the growth and intracellular nitrogen of Saccharina latissima through cultivating sporophytes in land-based tanks with four different combinations of high/low light and high/low nutrient supply over an experimental period of 20 days. The results revealed that the mean growth rate and the intracellular nitrogen component of the sporophytes were positively related to the external nitrate concentration during the experimental period, indicating that Saccharina latissima requires high nutrient concentration to maintain a rapid growth (Jevne, Forbord & Olsen, 2020).

Bokn et al. (2003) conducted a nutrient loading experiment on intertidal fucoids. Within three years of the experiment, no significant effect was observed in the communities. However, four to five years into the experiment, a shift occurred from perennials to ephemeral algae. Although Bokn et al. (2003) focused on fucoids, the results could indicate that long-term (>4 years) nutrient loading can result in a community shift to ephemeral algae species. Disparities between the findings of the aforementioned studies are likely to be related to the level of organic enrichment.

Johnston & Roberts (2009) conducted a meta-analysis, which reviewed 216 papers to assess how a variety of contaminants (including sewage and nutrient loading) affected six marine habitats (including subtidal reefs). A 30 to 50% reduction in species diversity and richness was identified from all habitats exposed to the contaminant types. Johnston & Roberts (2009), however, also highlighted that macroalgal communities were relatively tolerant to contamination, but that contaminated communities could have low diversity assemblages dominated by opportunistic and fast-growing species (Johnston & Roberts, 2009).

Sensitivity assessment. Although short-term exposure (<4 years) to nutrient enrichment may not affect seaweeds directly, indirect effects such as turbidity may significantly affect photosynthesis and result in reduced growth and reproduction and increased competition form fast growing but ephemeral species. However, the above evidence reported that the growth and productivity of Saccharina latissima depended on high nutrient levels and could benefit from nutrient enrichment, but did not report mortality due to nutrient enrichment. Hence, this biotope is considered to be 'Not sensitive'.

Organic enrichment

Benchmark. A deposit of 100 gC/m²/yr. Further detail

Evidence

Areas with high nutrient loading will sustain rapid macroalgal growth during the summer (Davison, Andrews & Stewart, 1984 cited in Kerrison et al., 2015), and where nutrient loading is lower, high water flow increases the nutrient uptake rate of macroalgae by refreshing the boundary layer (Wheeler & North (date) cited in Kerrison et al., 2015), so maximal growth rates can be sustained. It has been shown in Saccharina latissima that 10 μmol/l of nitrate is required to maximise growth rate and leads to internal storage for later use (Chapman, Markham & Lüning, 1978 cited in Kerrison et al., 2015). Boderskov et al. (2016) noted how Saccharina latissima grown under high nutrient availability in Denmark fulfils a higher degree of nutrient bioremediation, and has an improved biomass quality in regard to increased concentrations of pigments and nitrogen-rich compounds.

Conolly & Drew (1985) found Saccharina latissima sporophytes had relatively higher growth rates when in close proximity to a sewage outlet in St Andrews, UK, compared to other sites along the east coast of Scotland. At St Andrews, nitrate levels were 20.22 µM, which represents an approx. 25% increase compared to other sites (approx. 15.87 µM). Read et al. (1983) reported that after the installation of a new sewage treatment works, which reduced the suspended solid content of liquid effluent by 60% in the Firth of Forth, Saccharina latissima became abundant where previously it had been absent.Although short-term exposure (<4 years) to organic enrichment may not affect seaweeds directly, indirect effects such as turbidity may significantly affect photosynthesis (Read 1983), and result in reduced growth and reproduction and increased competition form fast growing but ephemeral species.

The association of fish and shellfish mariculture can also lead to an increased growth rate of macroalgae, while removing excess nutrients from the environment (Sanderson et al, 2008, Sanderson et al., 2012 and Wang et al., 2014 cited in Kerrison et al., 2015). Handå et al. (2013) reported Saccharina latissima sporophytes grew approx. 1% faster per day when in close proximity to Norwegian salmon farms, where elevated ammonium could be readily absorbed by sporophytes. However, experimentation in Denmark did not show any benefit in terms of growth, nitrogen, phosphorus, or amino acid content of Saccharina latissima cultured in proximity to fish and mussel aquaculture (Marinho, Holdt & Angelidaki, 2015 and Marinho et al., 2015 cited in Kerrison et al., 2015).

Rugiu et al. (2021) exposed Saccharina latissima to natural seawater, water enriched to levels of ammonium and nitrate simulating finfish cage waste (test IMTA1), and a combination of such enrichment with natural effluents coming from mussels (test IMTA2). The Saccharina latissima biomass was higher and produced elevated total organic content when exposed to both IMTA1 and IMTA2 nutrient scenarios, including a significant enhancement in pigment content only when algae were exposed to the strongest enrichment (IMTA2) (Rugiu et al., 2021). In addition, the photosynthetic responses in terms of relative electron transfer rate, PSII (photosystem II) saturation irradiance, total nitrogen content, and the content of chlorophyll-a and fucoxanthin were also positively affected by both IMTA1 and IMTA2 (Rugiu et al., 2021). Rugiu et al. (2021) concluded that Saccharina latissima showed a significant physiological response to nutrient enrichment mimicking aquaculture settings, as well as the benefit of added nutrients through a boost in photosynthetic activity that leads to higher kelp biomass and pigment production.

Fales et al. (2023) compared the physiological responses of Saccharina latissima sporophytes to high temperature stress (low: 9 and 13°C, moderate: 15 and 16°C, and warm: 21°C) and nitrogen limitation (low: 1 to 3 μM vs. high: >10 μM) over eight to nine days. Saccharina latissima responded negatively to elevated temperatures, but not to low nitrogen levels. Blades of Saccharina latissima showed signs of metabolic stress and reduced growth in the warmest temperature treatment (21°C), at both high and low nitrogen levels, suggesting that Saccharina latissima is susceptible to thermal stress over short time periods, and that nutrient additions may actually reduce kelp performance at supra-optimal temperatures (Fales et al., 2023).

Jevne, Forbord & Olsen (2020) examined how differences in light conditions and nutrient availability affect the growth and intracellular nitrogen of Saccharina latissima through cultivating sporophytes in land-based tanks with four different combinations of high/low light and high/low nutrient supply over an experimental period of 20 days. The results revealed that the mean growth rate and the intracellular nitrogen component of the sporophytes were positively related to the external nitrate concentration during the experimental period, indicating that Saccharina latissima requires high nutrient concentration to maintain a rapid growth (Jevne, Forbord & Olsen, 2020).

Bokn et al. (2003) conducted a nutrient loading experiment on intertidal fucoids. Within three years of the experiment, no significant effect was observed in the communities. However, four to five years into the experiment, a shift occurred from perennials to ephemeral algae. Although Bokn et al. (2003) focused on fucoids, the results could indicate that long-term (>4 years) nutrient loading can result in a community shift to ephemeral algae species. Disparities between the findings of the aforementioned studies are likely to be related to the level of organic enrichment.

Johnston & Roberts (2009) conducted a meta-analysis, which reviewed 216 papers to assess how a variety of contaminants (including sewage and nutrient loading) affected six marine habitats (including subtidal reefs). A 30 to 50% reduction in species diversity and richness was identified from all habitats exposed to the contaminant types. Johnston & Roberts (2009), however, also highlighted that macroalgal communities were relatively tolerant to contamination, but that contaminated communities could have low diversity assemblages dominated by opportunistic and fast-growing species (Johnston & Roberts, 2009).

Sensitivity assessment. Although short-term exposure (<4 years) to nutrient enrichment may not affect seaweeds directly, indirect effects such as turbidity may significantly affect photosynthesis and result in reduced growth and reproduction and increased competition form fast growing but ephemeral species. Therefore, Resistance has been assessed as ‘Medium’, and resilience as ‘High’. Sensitivity has been assessed as ’Low’.

Physical loss (to land or freshwater habitat)

Benchmark. A permanent loss of existing saline habitat within the site. Further detail

Evidence

All marine habitats and benthic species are considered to have a resistance of ‘None’ to this pressure and to be unable to recover from a permanent loss of habitat (resilience is ‘Very Low’).  Sensitivity within the direct spatial footprint of this pressure is therefore ‘High’. Although no specific evidence is described confidence in this assessment is ‘High’, due to the incontrovertible nature of this pressure.

Physical change (to another seabed type)

Benchmark. Permanent change from sedimentary or soft rock substrata to hard rock or artificial substrata or vice-versa. Further detail

Evidence

If sediment were replaced with rock or artificial substrata, this would represent a fundamental change to the biotope (Macleod et al., 2014). All the characterizing species within this biotope can grow on rock biotopes (Birkett et al., 1998; Connor et al., 2004), however SS.SMp.KSwSS are by definition sediment biotopes and introduction of rock would change them into a rock based habitat complex, and the biotope would be lost

Sensitivity assessment. Resistance to the pressure is considered ‘None’, and resilience ‘Very low’. Sensitivity has been assessed as ‘High’

Physical change (to another sediment type)

Benchmark. Permanent change in one Folk class (based on UK SeaMap simplified classification). Further detail

Evidence

SS.SMp.KSwSS are sediment based biotopes. Stabilised cobbles, pebbles, gravel and shell fractions provide a substrate for macro-algae to dominate the community (Connor et al., 2004). An increase in the dominance of smaller sediment fractions e.g. sand and/or mud will likely smoother the existing biotope, inhibit successive re-colonisation of macro-algae and/or increase the sediment scour.

Sensitivity assessment. Resistance has been assessed as ‘None’, resilience as Very low (the pressure is a permanent change), and sensitivity as High. 

Habitat structure changes - removal of substratum (extraction)

Benchmark. The extraction of substratum to 30 cm (where substratum includes sediments and soft rock but excludes hard bedrock). Further detail

Evidence

SS.SMp.KSwSS.SlatR (plus sub-biotopes), SS.SMp.KSwSS.SlatCho can be found on a varied mixture of sediment and rock fractions. Extraction of substratum to 30 cm is likely to remove small sediment fractions (e.g. gravel) and may mobilize the remaining larger rock fractions (e.g. boulders) causing high mortality within the resident community. All characterizing species have rapid growth rates and are likely to recover within 2 years.

Sensitivity assessment. Resistance has been assessed as ‘None’, Resilience as ‘High’. Sensitivity has been assessed as ‘Medium’.

Abrasion / disturbance of the surface of the substratum or seabed

Benchmark. Damage to surface features (e.g. species and physical structures within the habitat). Further detail

Evidence

Abrasion of the substratum e.g. from bottom or pot fishing gear, cable laying etc. may cause localised mobility of the substrata and mortality of the resident community. The effect would be situation dependent, however, if bottom fishing gear were towed over a site it may mobilise a high proportion of the rock substrata and cause high mortality in the resident community.

Sensitivity assessment. Resistance has been assessed as ‘None’, Resilience as ‘High’. Sensitivity has been assessed as ‘Medium’.

Penetration or disturbance of the substratum subsurface

Benchmark. Damage to sub-surface features (e.g. species and physical structures within the habitat). Further detail

Evidence

Penetration and/or disturbance of the substrate below the surface of the seabed, may cause localised mobility of the substrata and mortality of the resident community.

Sensitivity assessment. Resistance has been assessed as ‘None’, Resilience as ‘High’. Sensitivity has been assessed as ‘Medium’.

Changes in suspended solids (water clarity)

Benchmark. A change in one rank on the WFD (Water Framework Directive) scale e.g. from clear to intermediate for one year. Further detail

Evidence

Suspended Particle Matter (SPM) concentration has a positive linear relationship with sub-surface light attenuation (Kd) (Devlin et al., 2008). Also, although short-term exposure (<4 years) to organic enrichment may not affect seaweeds directly, indirect effects such as turbidity may significantly affect photosynthesis (Read 1983), and result in reduced growth and reproduction and increased competition form fast growing but ephemeral species.

Light availability and water turbidity are principal factors in determining the depth range at which macro-algae can be found (Birkett et al., 1998b). Light penetration influences the maximum depth at which laminarians can grow, and it has been reported that laminarians grow at depths at which the light levels are reduced to 1% of incident light at the surface. Maximal depth distribution of laminarians therefore varies from 100 m in the Mediterranean to only 6 to 7 m in the silt-laden German Bight. In Atlantic European waters, the depth limit is typically 35 m. In very turbid waters, the depth at which kelp is found may be reduced, or in some cases excluded completely (e.g. Severn Estuary), because of the alteration in light attenuation by suspended sediment (Lüning, 1990; Birkett et al. 1998b). Laminarians show a decrease of 50% photosynthetic activity when turbidity increases by 0.1/m (light attenuation coefficient =0.1-0.2/m; Staehr & Wernberg, 2009).

Sensitivity Assessment. A decrease in turbidity is likely to support enhanced growth (and possible habitat expansion) and is therefore not considered in this assessment. An increase in water turbidity is likely to primarily affect photosynthesis, therefore growth and density of the canopy-forming seaweeds. Resistance to this pressure is defined as ‘Low’ and resilience to this pressure is defined as ‘High’ at the benchmark level due to the scale of the impact. Hence, this biotope is regarded as having a sensitivity of ‘Low‘.

Smothering and siltation rate changes (light)

Benchmark. ‘Light’ deposition of up to 5 cm of fine material added to the seabed in a single discrete event. Further detail

Evidence

Smothering by sediment e.g. 5 cm material during a discrete event, is unlikely to damage mature examples of Saccharina latissima and Chorda filum but may provide a physical barrier to zoospore settlement and therefore could negatively impact on recruitment processes (Moy & Christie, 2012). Laboratory studies showed that kelp and gametophytes can survive in darkness for between 6-16 months at 8 °C and would probably survive smothering by a discrete event and once returned to normal conditions gametophytes resumed growth or maturation within 1 month (Dieck, 1993).

SS.SMp.KSwSS biotopes are all recorded in moderately strong tidal streams to negligible (≤1.5 m/sec) (Connor et al., 2004). In tidally exposed biotopes deposited sediment is unlikely to remain for more than a few tidal cycles (due to water flow or wave action). In sheltered biotopes deposited sediment could remain however are unlikely to remain for longer than a year.

Sensitivity assessment. Resistance has been assessed as ‘High’, resilience as ‘High’. Sensitivity has been assessed as ‘Not Sensitive’.

Smothering and siltation rate changes (heavy)

Benchmark. ‘Heavy’ deposition of up to 30 cm of fine material added to the seabed in a single discrete event. Further detail

Evidence

Smothering by sediment e.g. 30 cm material during a discrete event, is unlikely to damage mature examples of Saccharina latissima and Chorda filum but may provide a physical barrier to zoospore settlement and therefore could negatively impact on recruitment processes (Moy & Christie, 2012). Laboratory studies showed that kelp and gametophytes can survive in darkness for between 6-16 months at 8°C and would probably survive smothering by a discrete event and once returned to normal conditions gametophytes resumed growth or maturation within 1 month (Dieck, 1993).

SS.SMp.KSwSS biotopes are all recorded in moderately strong tidal streams to negligible (≤1.5 m/sec) (Connor et al., 2004). In tidally exposed biotopes deposited sediment is unlikely to remain for more than a few tidal cycles (due to water flow or wave action). In sheltered biotopes deposited sediment could remain however are unlikely to remain for longer than a year.

Sensitivity assessment. Resistance has been assessed as ‘Medium’, resilience as ‘High’. Sensitivity has been assessed as ‘Low’.

Litter

Benchmark. The introduction of man-made objects able to cause physical harm (surface, water column, seafloor or strandline). Further detail

Evidence

Not assessed.

Electromagnetic changes

Benchmark. A local electric field of 1 V/m or a local magnetic field of 10 µT. Further detail

Evidence

Evidence on the effect of electromagnetic fields (EMFs) on benthic organisms is still severely lacking. Some studies have investigated the effect of anthropogenically induced EMFs on benthic invertebrates at intensities ranging between 2 nT and 40 mT, which is often much higher than in-situ measurements from subsea cables. While some report changes to behaviour, physiology, reproduction, development, immunology, cytotoxicity and orientation, others demonstrate no effect from exposure to the EMF (Albert et al., 2020; Hutchison et al., 2020; MacKenzie, 2024), depending on the study species and duration and intensity of exposure. There have been no studies investigating the effect of EMFs at the population or community level for benthic organisms. No studies have examined the effect of EMFs on Saccharina latissima or Chorda filum. There is 'Insufficient evidence' on which to base an assessment of the likely sensitivity of this biotope to EMFs.

Underwater noise changes

Benchmark. MSFD indicator levels (SEL or peak SPL) exceeded for 20% of days in a calendar year. Further detail

Evidence

Not relevant

Introduction of light or shading

Benchmark. A change in incident light via anthropogenic means. Further detail

Evidence

The availability of light is highly spatio-temporally variable, and its oversupply can be a major threat to macroalgal survival (Airoldi & Beck, 2007 cited in Kerrison et al., 2015). If too much light is absorbed by kelp, the excess energy can inhibit photosynthesis (Dring, Wagner & Luning, 2001 cited in Kerrison et al., 2015) and may lead to cellular damage and death of the organism. This sets an upper depth limit for many species. Conversely, sufficient photosynthetically active radiation must be supplied to sustain growth, so setting a lower depth limit. In very clear waters, some kelps can grow down to 30 to 40 m (Smale et al., 2013; Khan et al., 2018), while in waters carrying suspended sediment, light penetration declines quickly, leading to a shallow limit of less than a metre. The optimum depth for growth of Saccharina latissima has been reported as 9 to 12 m in Maine, USA (Boden, 1979 cited in Kerrison et al., 2015), 5 m in mid-Norway (Handå et al., 2013 cited in Kerrison et al., 2015), or only 1.5 to 3 m in Scotland (Kerrison et al., 2015). Young Saccharina latissima sporophytes appear to have similar light requirements and tolerance as Laminaria digitata (Han & Kain, 1996 cited in Kerrison et al., 2015). While adults are light saturated at around 215 μmol m²/s and have their maximum photosynthetic rate at 200 μmol m²/s (Bartsch et al., 2008 cited in Kerrison et al., 2015). One or two hours of light at 500 to 700 μmol m²/s leads to significant dynamic photoinhibition and photodamage, with young sporophytes being more susceptible than adults (Bruhn & Gerard, 1996 and Hanelt, Wiencke & Karsten, 1997 cited in Kerrison et al., 2015). High light exposure can therefore lead to the death of thallus tissue and the loss of biomass (Kerrison et al., 2015).

Ebbing et al. (2020) studied how light and biomass density influence the reproduction of delayed Saccharina latissima gametophytes over 21 days. They reported that reproductive success decreased at high light intensities (≥80 µmol photons/m²/s) across all light qualities and that optimal reproduction occurred at light intensities between 14.2 µmol and 25.7 µmol photons/m²/s (Ebbing et al., 2020). In addition, white light led to the highest reproductive success under optimal Initial Gametophyte Density conditions, while blue light resulted in the lowest reproductive success, especially at higher intensities (Ebbing et al., 2020). Red light at low intensity (5 µmol photons/m²/s) significantly inhibited reproduction, but higher intensities of red light improve reproductive success (Ebbing et al., 2020). ​Finally, Photosynthetically Usable Radiation (PUR), which integrates light intensity and quality, is a strong abiotic factor regulating reproduction, and reproductive success decreases when PUR exceeds 26.8 µmol photons/m²/s, regardless of light quality (Ebbing et al., 2020). In a further study, Ebbing et al. (2021) also observed optimal reproduction of Saccharina latissima at lower temperatures (10.2°C), but at high light intensities (≥29 µmol photons/m²/s), and at higher temperatures (≥12.6°C) at lower light intensities (≤15 µmol photons/m²/s); highlighting both spring and autumn as the optimal seasons for Saccharina latissima reproduction. Furthermore, Ebbing et al. (2021) demonstrated that delayed gametophytes of Saccharina latissima could reliably reproduce sexually after more than a year of vegetative growth, depending on the effects of light intensity and temperature. These findings suggest that both the quantity and quality of light, along with temperature, play critical roles in regulating the reproduction of delayed Saccharina latissima gametophytes.

Jevne, Forbord & Olsen (2020) examined how differences in light conditions and nutrient availability affect the growth and intracellular nitrogen of Saccharina latissima through cultivating sporophytes in land-based tanks with four different combinations of high/low light and high/low nutrient supply over an experimental period of 20 days. Although the results revealed that the mean growth rate and the intracellular nitrogen components of the sporophytes were positively related to the external nitrate concentration during the experimental period, the authors note how they saw no significant difference between the high light and the low light treatments (Jevne, Forbord & Olsen, 2020). Jevne, Forbord & Olsen (2020) did go on to highlight how Saccharina latissima grown between 10 and 15°C at a high light intensity of 250 μmol m²/s showed a 50% lower growth rate compared with kelps grown at 110 μmol m²/s, which was found to be the optimum for photon flux in this temperature interval (citing Fortes & Lüning, 1980). 

De Jong et al. (2021) also studied the effect of nutrient availability and light intensity on the sterol content of Saccharina latissima over a five-week period, subjected to a nutrient-replete and nutrient-depleted regime, then followed by the introduction of light-limited and light-saturated conditions in the sixth week. No significant inter-treatment differences were found in the sterol content in weeks one to five. However, significant intra-treatment differences were found in weeks three to five, regardless of nutrient treatment, wherein the fucosterol, 24-methylenecholesterol, and squalene contents of both treatment groups were found to correlate inversely with photosynthetic performance (de Jong et al., 2021). Concentrations of all other sterolic components increased with increasing irradiance and low nutrient conditions, while decreasing or remaining unchanged with increasing irradiance and high nutrient conditions (de Jong et al., 2021). From their data, de Jong et al. (2021) suggests, within their timeframe, the sterol content of Saccharina latissima is unaffected by nutrient availability alone but changes with combined alterations in irradiance and nutrient availability.

Niedzwiedz et al. (2024) studied the response of Saccharina latissima to realistic Arctic summer heatwave scenarios (4 to 10°C) under low- and high-light conditions (3 and 120 μmol photons/m²/s) for 12 days. They found that high-light caused physiological stress in Saccharina latissima (e.g., lower photosynthetic efficiency of photosystem II), which was enhanced by cold and mitigated by warm temperatures, and under low-light conditions, there was no temperature response, likely due to light limitation (Niedzwiedz et al., 2024). However, Saccharina latissima acclimated to light variations by adjusting its chlorophyll-a concentration, meeting cellular energy requirements (Niedzwiedz et al., 2024). Cobos et al. (2025) also studied the response of Arctic Saccharina latissima to light. They collected samples from Kongsfjorden, Svalbard, in early February and incubated them in dim light (6 μmol photons/m²/s) and dark (complete darkness) conditions for seven days. Saccharina latissima responded to light by decreasing its partial derivative carbon13 values, indicating some activation of its carbon concentrating mechanism, and increased its maximum photosynthetic electron transport rate; overall, showing that dim light had the potential to trigger photosynthetic metabolism and growth as early as February (Cobos et al., 2025).

Müller, Wiencke & Bischof (2008) found that elevated temperatures can exacerbate stress from ultraviolet radiation from sunlight. They investigated the combined effects of temperature and light quality on early life stages of Laminaria digitata and Saccharina latissima from Arctic (Spitsbergen) and temperate (Helgoland) populations. Temperature treatments ranged from 2°C to 18°C, representing Arctic summer conditions and North Sea summer extremes. For Laminaria digitata, Arctic populations germinated well at 2 to 12°C but failed at 18°C, while Helgoland populations showed optimal germination at 7 to 18°C. Saccharina latissima exhibited very low germination in Arctic populations (8 to 35%) and complete inhibition at 18°C, whereas temperate populations maintained high germination (85 to 92%) across all temperatures. UV-B radiation was the most damaging factor, reducing germination by up to 99% in Arctic Laminaria digitata and 74 to 90% in Arctic Saccharina latissima, and strongly inhibiting egg release (from 19 to 34 eggs mm² under normal light to 1.5 to 4 eggs mm² under UV-B). UV-A occasionally enhanced gametogenesis at moderate temperatures but did not offset UV-B damage. Overall, more light (UV exposure) combined with higher temperatures produced the greatest negative effects, while low light and moderate temperatures favoured Arctic populations. These findings indicate that warming exacerbates UV-B stress and severely limits recruitment (Müller, Wiencke & Bischof, 2008).

There is now a growing body of evidence to show that artificial light at night (ALAN) is widespread in the marine environment, with biologically relevant levels of light penetrating to depths of up to 50 m (Davies et al., 2020; Smyth et al., 2021). In other seaweeds, ALAN has been shown to change the timing of Ascophyllum nodosum and Fucus serratus reproduction, with receptacles (the reproductive tissues of fucoid macroalgae) continuing to ripen into the winter months instead of peaking in the summer (Moyse et al., 2025). This change in the timing of reproduction could result in gametes being released during suboptimal conditions, such as winter storms, and therefore reduce fertilization success. Reduced recruitment may lead to shifts in macroalgal assemblages in favour of species that are less sensitive to ALAN, such as Fucus vesiculosus, which seems to be unaffected (Moyse et al., 2025). ALAN can also vary significantly on small spatial scales and therefore affect some macroalgal forests more than others, even if they are close to one another. It is therefore possible that ALAN could cause changes in macroalgal assemblages over time.

Lastly, shading of the biotope (e.g. by the construction of a pontoon, pier, etc.) could adversely affect the biotope in areas where the water clarity is also low, and tip the balance to shade-tolerant species, resulting in the loss of the biotope directly within the shaded area, or a reduction in seaweed abundance.

Sensitivity assessment. Although both the addition and removal of light from the environment can have both positive and negative effects on kelps, such as inducing stress (Müller, Wiencke & Bischof, 2008; Niedzwiedz et al., 2024), affecting growth (Jevne, Forbord & Olsen, 2020), or influencing reproduction (Müller, Wiencke & Bischof, 2008; Ebbing et al., 2020; 2021), the evidence for the influence of artificial light on Saccharina latissima is limited. The addition of shading in the environment could reduce growth and increase stress, as sufficient photosynthetically active radiation must be supplied to sustain kelp growth, and conversely, from the removal of shade, too much light absorbed by kelp can inhibit photosynthesis and may lead to cellular damage and death of the organism (Kerrison et al., 2015). Therefore, resistance is probably 'Low', with a 'High' resilience and a sensitivity of 'Low', albeit with 'low' confidence due to the lack of direct evidence.

Barrier to species movement

Benchmark. A permanent or temporary barrier to species movement over ≥50% of water body width or a 10% change in tidal excursion. Further detail

Evidence

Not relevant. This pressure is considered applicable to mobile species, e.g. fish and marine mammals rather than seabed habitats. Physical and hydrographic barriers may limit the dispersal of spores.  But spore dispersal is not considered under the pressure definition and benchmark.

Death or injury by collision

Benchmark. Injury or mortality from collisions of biota with both static or moving structures due to 0.1% of tidal volume on an average tide, passing through an artificial structure. Further detail

Evidence

Not relevant. Collision from grounding vessels is addressed under abrasion above.

Visual disturbance

Benchmark. The daily duration of transient visual cues exceeds 10% of the period of site occupancy by the feature. Further detail

Evidence

Not relevant

Genetic modification & translocation of indigenous species

Benchmark. Translocation of indigenous species or the introduction of genetically modified or genetically different populations of indigenous species that may result in changes in the genetic structure of local populations, hybridization, or change in community structure. Further detail

Evidence

No evidence for the translocation of Saccharina latissima or Chorda filum was found.

Introduction of microbial pathogens

Benchmark. The introduction of relevant microbial pathogens or metazoan disease vectors to an area where they are currently not present (e.g. Martelia refringens and Bonamia, Avian influenza virus, viral Haemorrhagic Septicaemia virus). Further detail

Evidence

Little is currently known about diseases in kelp, or seaweeds in general, although various causative agents have been implicated (Gachon et al., 2010 cited in Kerrison et al., 2015). The bacteria Pseudoalterom spp. and Alteromonas spp. are known to be responsible for some diseases (Egan et al., 2014 cited in Kerrison et al., 2015), but in numerous cases, the agent has not been identified. The prevalence of endophytic infection is known to be high in wild kelp populations (Ellertsdóttir & Peters, 1997 cited in Kerrison et al., 2015), and so there are concerns that pathogens may be transplanted with seaweed stocks, infecting nearby natural seaweed beds, and as physicochemical stress is often a trigger for outbreaks in cultivated kelp (FAO, 2015 cited in Kerrison et al., 2015), climate change impacts such as rising seawater temperatures may in the future lead to more severe disease impacts.

Laminarians may be infected by the microscopic brown alga Streblonema aecidioides. Infected algae show symptoms of Streblonema disease, i.e. alterations of the blade and stipe ranging from dark spots to heavy deformations and completely crippled thalli. Infection can reduce growth rates of host algae (Peters & Scaffelke, 1996). The marine fungi Eurychasma spp can also infect early life stages of Laminarians, however, the effects of infection are unknown (Müller et al., 1999).

Sensitivity assessment. The evidence of diseases in laminarians suggests that growth and, possibly, survival may be affected. Hence, resistance is assessed as ‘Medium’ to represent the possible loss of a proportion of the population, but with ‘Low’ confidence due to the lack of direct evidence of mortality in the dominant characteristic species. Hence, resilience is assessed as ‘High’ and sensitivity to the introduction of microbial pathogens is assessed as ‘Low’.

Removal of target species

Benchmark. Removal of species targeted by fishery, shellfishery or harvesting at a commercial or recreational scale. Further detail

Evidence

This pressure has been assessed as ‘Not relevant’.

There has been recent commercial interest in Saccharina lattisma as a consumable called “sea vegetables” (Birket et al., 1998). However, Saccharina lattissima sporophytes are typically matured on ropes (Handå et al 2013) and not directly extracted from the seabed, as with Laminaria hyperborea (Christie et al., 1998). No evidence has been found for commercial extraction of Chorda filum.

Removal of non-target species

Benchmark. Removal of features or incidental non-targeted catch (by-catch) through targeted fishery, shellfishery or harvesting at a commercial or recreational scale. Further detail

Evidence

The main grazers of natural kelp forests are benthic invertebrates such as sea urchins, snails, abalone and small crustaceans. Natural kelp beds can be decimated by outbreaks of these grazers, although these may be prevented by top-down pressure from fishing (Johnson et al., 2013) or predators such as carnivorous fish or otters (Estes & Palmisano, 1974 and Davenport & Anderson, 2007 cited in Kerrison et al., 2015). The removal of sea urchins could lead to increases in kelp abundances (Miller et al., 2024b).

In addition, low-level disturbances (e.g. solitary anchors) are unlikely to cause harm to the biotope as a whole, due to the impact’s small footprint. Thus, evidence to assess the resistance of SS.SMp.KSwSS.SlatR (plus sub-biotopes), SS.SMp.KSwSS.SlatCho to non-targeted removal is limited. It is assumed that incidental non-targeted catch (e.g. by trawls or dredges) could mobilise sediment, remove large kelp species, overturn boulders and cobbles and bury smaller seaweeds and cause high mortality within the affected area.

Sensitivity assessment. Resistance has been assessed as ‘None’, Resilience as ‘High’. Sensitivity has been assessed as ‘Medium’.

The American slipper limpet, Crepidula fornicata

Evidence

The American slipper limpet Crepidula fornicata was introduced to the UK and Europe in the 1870s from the Atlantic coasts of North America with imports of the eastern oyster Crassostrea virginica. It was recorded in Liverpool in 1870 and the Essex coast in 1887 to 1890, and has spread into waters around mainland Europe (Blanchard, 1997, 2009; Bohn et al., 2012, 2013a, 2013b, 2015; De Montaudouin et al., 1999, 2018; Hinz et al., 2011; Helmer et al., 2019; McNeill et al., 2010; Powell-Jennings & Calloway, 2018; Preston et al., 2020; Stiger-Pouvreau & Thouzeau, 2015). It ranges from the Baltic Sea, the Kattegat and Skagerrak, the North Sea coasts of the UK, Germany, and Belgium, through the English Channels and into the Irish sea coasts of Ireland and south Wales with records in east and west Scotland, Northern Ireland, northwest France, Spain and south into the Mediterranean (NBN, 2024; OBIS, 2025).

Abundances at its northern and southern extremes may be low, but densities in the UK and France are often over 1000 /m², and it may carpet the seafloor in the Solent and Essex. In the UK, it was reported to reach abundances of >1000 /m² (max. 2,748 /m²) in the Milford Harbour Waterway (Bohn et al., 2012), 84 /m² in Portsmouth, 174 /m² in Langstone and 306 /m² in Chichester harbours in 2017 (Helmer et al., 2019). In France, it has been reported to reach >4,700 /m² in the Bay of Marennes-Oleron, 11.6 tonnes/ha in the Bay of Mont-Saint-Michel, 8.2 tonnes/ha in the Bay of Brest and 2.8 tonnes/ha in the Bay of Saint-Brieuc (Blanchard, 2009; Bohn et al., 2012, 2015; Powell-Jennings & Calloway, 2018).

Its density and ability to spread within and between sites (e.g., bays) depend on the availability of suitable habitat, competition with other species, larval retention within the site, human activities (e.g., dredging), and seasonal temperatures, particularly in the intertidal zone. For example, the Crepidula fornicata population in the Bay of Mont-Saint-Michel grew by 50% between 1996 and 2004, covering 25% of the area at high density (51–100% cover), aided by local oyster farming and shellfish dredging (Blanchard, 2009). However, in Arcachon Bay, France, Crepidula fornicata was limited to only 155 tonnes in 1999 and 312 tonnes in 2011 (De Montaudouin et al., 2001, 2018). It was confined to muddy sediments, which accounted for approximately 8% of the bay and were colonized by Zostera beds. These areas represented just 0.4% of the suspension feeder biomass compared to the oyster Magallana gigas in the bay, and there was no indication of increasing biomass over a 12-year period. In addition, benthic trawling was prohibited in the bay (De Montaudouin et al., 2001, 2018). As a result, De Montaudouin et al. (2018) concluded that Crepidula fornicata was not invasive in the Bay of Arcachon.

Crepidula fornicata is recorded from shallow, sheltered bays, lagoons and estuaries or the sheltered sides of islands, in variable salinity (from 18 to 40 ppt), although it prefers around 30 ppt (Tillin et al., 2020). Larvae require hard substrata for settlement. It prefers muddy, gravelly, shell-rich substrata that include gravel, or shells of other Crepidula, or other species, e.g., oysters, and mussels. It is highly gregarious and seeks out adult shells for settlement, forming characteristic ‘stacks’ of adults. It has also been recorded from rock, artificial substrata, and Sabellaria alveolata reefs (Blanchard, 1997, 2009; Bohn et al., 2012, 2013a, 2013b, 2015; De Montaudouin et al., 2018; Hinz et al., 2011; Helmer et al., 2019; Powell-Jennings & Calloway, 2018; Preston et al., 2020; Stiger-Pouvreau & Thouzeau, 2015; Tillin et al., 2020).

In the eastern Solent harbours of Portsmouth, Langstone, and Chichester, 75% to 98% of Crepidula larvae settled on dead Crepidula shells, while ~4% settled on stone, 2.5% on live Crepidula, 0.3% oyster shell, 0.6% cockle shell, 0.3% winkle shell and 0.1% perwinkle shell (Preston et al., 2020). In the Milford Harbour Waterway, the highest densities of Crepidula were found in areas of sediment with hard substrata, e.g., mixed fine sediment with shell, or gravel or both, while Crepidula density increased as gravel cover increased in the subtidal, the reverse was found in the intertidal (Bohn et al., 2015). However, gravel formed the base of most stacks of Crepidula in the intertidal, which suggested that initial colonization occurred on available hard substrata (i.e., gravel) in the absence of adult shells of Crepidula. The availability of hard substrata (e.g., gravel) may only restrict initial colonization as higher densities of Crepidula function as substrata for subsequent colonization (Thieltges et al., 2004; Blanchard, 2009). Bohn et al. (2015) also noted that Crepidula density was low in areas of homogenous fine sediment and absent in areas dominated by boulders.

Bohn et al. (2015) suggested that wave action (exposure) probably prevented the establishment of large numbers of Crepidula in high-energy areas. However, Hinz et al. (2011) recorded Crepidula off the Isle of Wight in the English Channel, at ~60 m on rough ground in areas of high tidal flow. Tillin et al. (2020) suggested that the effect of oscillatory wave-mediated flow might have a greater effect on Crepidula than tidal flow, presumably due to mobilization of the substratum. Similarly, Crepidula was absent from sandy substrata in Swansea Bay but was most abundant in the shelter of the breakwater at Swansea east site (Powell-Jennings & Calloway, 2018).

Crepidula fornicata has been recorded from the lower intertidal to ~160 m in depth, but it is most common in the shallow subtidal and low water springs (Blanchard, 1997; Thieltges et al., 2003; Bohn et al., 2012, 2015; Hinz et al., 2011; OBIS, 2025; Tillin et al., 2020). Bohn et al. (2012, 2013a, 2013b, 2015) suggested that extreme conditions in the intertidal limited its upward distribution due to early post-settlement mortality. It reached its highest densities in the lower shore (below approx.0.7 m) and was absent from high tidal levels (approx.1.8 m) in the Milford Harbour Waterway (Bohn et al., 2015).

The density of Crepidula populations in northern Europe (Germany, Denmark, and Norway) are significantly lower (<100 /m²) than in southern waters. Thieltges et al. (2004) reported that the population of Crepidula was affected strongly by cold winters in the Wadden Sea. The winters of 2001 and 2003 resulted in ca 56-64% mortality of intertidal Crepidula and up to 97% on one mussel bed, compared to only 11-14% in southern areas without frost. Crepidula almost vanished from the Wadden Sea after the 1978/79 winter and took ten years to recover due to moderate winters, which regularly affected the population. Similarly, 25% mortality was observed in Crepidula populations on the south coast of the UK after the extreme 1962/63 winter (Crisp, 1964, Bohn et al., 2012). Thieltges et al. (2003) suggested that global warming may allow Crepidula populations become more abundant in northern Europe. Valdizan et al. (2011) noted that higher water temperatures between 2000 and 2001 and 2006 to 2007, together with elevated chlorophyll-a, corresponded to an increase in gametogenesis and the duration of broods in Crepidula population in Bournerf Bay, France. They suggested that rising temperatures in northern Europe could increase its reproductive success due to favourable breeding temperatures and increased phytoplankton (Valdizan et al., 2011).

Sensitivity assessment.

No evidence was found on the interaction of Crepidula and kelp and seaweed-dominated communities on sublittoral sediments. Tillin et al. (2020) suggested that Crepidula was unlikely to colonize kelp and seaweed-dominated communities on sublittoral sediment. De Montaudouin et al. (2001, 2018) noted that abundance of Crepidula was limited in areas of muddy sediment occupied by Zostera in Arcachon Bay, France, and concluded it was not invasive in the bay. Tillin et al. (2020) suggested that the sediment type (muds) and the sweeping of Zostera leaves probably limited Crepidula abundance and hence suggested that kelp and seaweeds could also limit colonization of sublittoral sediment by Crepidula, albeit with ‘Low’ confidence’.

However, the presence of kelp and other seaweeds in this biotope group (SS.SMp.KSwSS.SlatR) is dependent on the availability of hard substrata provided by the mixed sediment, or gravels, cobbles, shell, etc., in soft sediments (Connor et al., 2004). Hence, the suitability of the biotope for colonization by Crepidula probably varies depending on the underlying sediment type, and hence wave action, while its subsequent abundance may also be limited by the presence of kelps and other seaweeds.

Therefore, SS.SMp.KSwSS.SlatR.CbPb is dominated by cobbles and pebbles and occurs in moderately high-energy areas, which are seasonally disturbed (presumably by storms) and are probably unsuitable for colonization by Crepidula. Hence, resistance and resilience are assessed as ‘High’ and the biotope (SlatR.CbPB) as ‘Not sensitive’, based on expert judgement, albeit with low confidence.

SS.SMp.KSwSS.LsacR.Gv is more stable than SlatR.CbPb) and dominated by gravel and mixed sediments are suitable for Crepidula. Similarly, most of the remaining SlatR biotopes occur on sediments that include hard substrata (pebbles, cobbles, gravel and shell) (Connor et al., 2004), which could provide suitable attachment for Crepidula larvae and allow it to get a foothold. Therefore, a resistance of 'Medium' (some mortality, <25%) is suggested as a precaution, to represent colonization by Crepidula, although its abundance may be mitigated by the presence of kelps and seaweeds. Resilience is likely to be 'Very low’ as Crepidula would need to be removed to allow recovery. Hence, sensitivity is assessed as ‘Medium’ but with ‘Low’ confidence due to the lack of direct evidence.

The carpet sea squirt, Didemnum vexillum

Evidence

The carpet sea squirt Didemnum vexillum (syn. Didemnum vestitum; Didemnum vestum) is a colonial ascidian with rapidly expanding populations that have invaded most temperate coastal regions around the world (Kleeman, 2009; Stefaniak et al., 2012; Tillin et al., 2020). It is an ‘ecosystem engineer’ that can change or modify invaded habitats and alter biodiversity (Griffith et al., 2009; Mercer et al., 2009).

A lack of published descriptions and an incomplete historical record have led to the widespread misidentification of Didemnum vexillum, and it is often recorded as Didemnum spp. Hence, the native range of the species is not known conclusively (Lambert, 2009; Stefaniak et al., 2012; McKenzie et al., 2017; Holt, 2024). However, molecular data and limited historical evidence have suggested that the species may be native to Japan, with its native range possibly extending into continental Asia and north-western Pacific (Stefaniak et al., 2012; Tillin et al., 2020; Holt, 2024). Previously unrecorded populations of a colonial ascidian have been recently identified as Didemnum vexillum (Tillin et al., 2020).

Didemnum vexillum has colonized and established populations in the northeast Pacific, Canadian and USA coast; New Zealand; France, Spain, and the Wadden Sea, Netherlands; the Mediterranean Sea and Adriatic Sea (Bullard et al., 2007; Coutts & Forrest, 2007; Dijkstra et al., 2007; Valentine et al., 2007a; Valentine et al., 2007b; Lambert, 2009; Hitchin, 2012; Tagliapietra et al., 2012; Gittenberger et al., 2015; Vercaemer et al., 2015; Mckenzie et al., 2017; Cinar & Ozgul, 2023; Holt, 2024).

In the UK, Didemnum vexillum has colonized Holyhead marina and Milford Haven, Wales; the west coast of Scotland (marinas around Largs, Clyde, Loch Creran and Loch Fyne), South Devon (Plymouth, Yealm, and Dartmouth estuaries), the Solent, northern Kent, Essex, and Suffolk coasts (Griffith et al., 2009; Lambert, 2009; Hitchin, 2012; Minchin & Nunn, 2013; Bishop et al., 2015; Mckenzie et al., 2017; Tillin et al., 2020, Holt, 2024; NBN, 2024).

Didemnum vexillum has the ability to rapidly overgrow and displace on other sessile organisms such as other colonial ascidians (Ciona intestinalis, Styela clava, Ascidiella aspera, Botrylloides violaceus, Botryllus schlosseri, Diplosoma listerianium and Aplidium spp.), bryozoan, hydroids, sponges (Clione celata and Halichrondria sp.), anemone (Diadumene cincta), calcareous tube worms, eelgrass (Zostera marina), kelp (Laminaria spp. and Agarum sp.), green algae (Codium fragile subsp. fragile), red algae (Plocamium, Chondrus crispus and bush weed Agardhiella subulata), brown algae (Ascophyllum nodosum, Sargassum, Halidrys, Fucus evanescens and Fucus serratus), calcareous algae (Corallina officinalis), mussels (Mytilus galloprovincialis, Perna canaliculus and Mytilus edulis), barnacles, oysters (Magallana gigas, Ostrea edulis and Crassostrea virginica), sea scallops (Placopecten magellanicus), or dead shells (Dijkstra et al., 2007; Gittenberger, 2007; Valentine et al., 2007a; Valentine et al., 2007b; Griffith et al., 2009; Carman & Grunden, 2010; Dijkstra & Nolan, 2011; Groner et al., 2011; Hitchin, 2012; Tagliapietra et al., 2012; Minchin & Nunn, 2013; Gittenberger et al., 2015; Long & Groholz, 2015; Vercaemer et al., 2015).

Didemnum vexillum has been found colonizing the stipes of Laminaria spp. in the Gulf of Maine (Dijkstra et al., 2007) and in Norway (Legrand et al., 2025). However, it has not been recorded in sites exposed to wave action, that is, 'very wave exposed', 'wave exposed' and 'moderately wave exposed' (sensu MNCR, Hiscock, 1996), especially in the intertidal, where wave action is not ameliorated by depth (see Hiscock, 1983).

This species requires suitable hard substrata for successful settlement and the establishment of colonies. It can grow quickly and can establish large colonies of dense encrusting mats on a variety of hard substrata (Valentine et al., 2007a; Griffith et al., 2009; Lambert, 2009; Groner et al., 2011; Cinar & Ozgul, 2023). Mats can be up to several meters in area, covering large portions of the seafloor (Mercer et al., 2009). Gittenberger (2007) stated that invasive Didemnum sp. was a threat to native ecosystems by its ability to overgrow virtually all hard substrata present. Suitable hard substrata can include rocky substrata such as bedrock, gravel, pebble, cobble, or boulders (Tillin et al., 2020). Didemnum vexillum has been reported colonizing these types of hard substrata in the USA, Canada, northern Kent and the Solent (Bullard et al., 2007; Valentine et al., 2007a; Valentine et al., 2007b; Hitchin, 2012; Vercaemer et al., 2015; Tillin et al., 2020). It is therefore likely that the substrate in this biotope is suitable for Didemnum vexillum colonisation. In addition, the depth range at which Laminaria hyperborea biotopes are found (0 to 30 m) overlaps with the depth range that is suitable for Didemnum vexillum colonization. Didemnum vexillum has been recorded from less than 1 m to at least 81 m deep (Bullard et al., 2007; Tagliapietra et al., 2012; Tillin et al., 2020).

Didemnum vexillum tolerates a wide range of environmental conditions, including temperature and salinity (Herborg et al., 2009; Tillin et al., 2020). Didemnum vexillum can withstand a wide range of salinities from 20 to 44 ppt, is commonly found in marine waters around 33 ppt, but is unable to survive in salinities below 20 ppt (Bullard & Whitlatch, 2009; Groner et al., 2011; Tillin et al., 2020). It has been recorded in estuarine conditions and tidal lagoons (Dijistra et al., 2007; Tillin et al., 2020). In the Lagoon of Venice, Mediterranean, Didemnum vexillum is found in a mean salinity value of 30 PSU. It was absent in low salinity, such as the estuary and around the salt marshes, but well established in the euhaline and tidally well-flushed zones of the Lagoon of Venice (Tagliapietra et al., 2012). Similar results were found in Connecticut and Rhode Island, where Didemnum vexillum was not found in environments with salinity less than 20 ppt (Bullard & Whitlatch, 2009). However, in the Wadden Sea, colonies of Didemnum vexillum were abundant in salinities between 17.91 and 25.97 ppt (Gittenberger, 2007; Gittenberger et al., 2015).

Didemnum vexillum is a temperate species that can survive a broad temperature range of -2 to 24°C, with an upper survival limit suggested to be 25°C (Bullard et al., 2007; Valentine et al., 2007a; Herborg et al., 2009; Kleeman, 2009; McKenzie et al., 2017; Holt, 2024). It thrives best at 14 to 20°C, with optimal growth temperature between 14 and 18°C during summer months (May, June, September, October) (Gittenberger, 2007; Kleeman, 2009; McKenzie et al., 2017).

Reinhart et al. (2012) examined the effects of water flow and hydrodynamics on the encrusting and tendril forms of Didemnum vexillum. They reported that a current speed of approx. 7.6 m/s was required to induce fragmentation of tendrils, but that natural tidal flow alone was insufficient to cause fragmentation of tendrils. They suggested that rare instances of wave action, such as storms that resulted in wave orbital velocities of ca 8 m/s or (more likely) human activity, could cause fragmentation of tendrils.

Reinhart et al. (2012) noted that the tensile strength of Didemnum vexillum was an order of magnitude higher than that of Botrylloides sp. and was similar to that of Alyconium digitatum. Alyconium digitatum is reported from sheltered to very wave-exposed conditions, but in the sublittoral. Reinhart et al. (2012) also suggested that seasonal changes in the condition of Didemnum vexillum reduced the tensile strength of colonies and were associated with the period of greater larval production, and implied that fragmentation aided dispersal.

The oscillatory nature of wave-mediated water flow (wave orbital velocities) combined with wave pressure in the lacerating zone, where breaking waves cause multidirectional strong water movement (Hiscock, 1983), would probably dislodge and break up Didemnum vexillum colonies, prevent them from forming suffocating mats, and restrict the colonies to crevices and overhangs. However, it is unclear if moderately wave-exposed conditions would be adequate to prevent Didemnum vexillum from developing extensive mats in the summer months when wave action is typically reduced. Hitchin (2012) suggested that the presence of Didemnum vexillum in Whitstable, Kent, was contrary to its then-known habitat preferences.

Considering other epiphytes, such as bryozoans, mussels, and other seaweeds, their overgrowth of the cultivated thalli on Saccharina latissima has led to its degradation and tissue loss during late spring or early summer (Marinho, Holdt & Angelidaki, 2015 cited in Kerrison et al., 2015; Kerrison et al., 2015).

Sensitivity assessment. There is no evidence of Didemnum vexillum colonizing this biotope in the UK. However, it has been recorded in similar kelp habitats in Norway (Järnegren et al., 2023). Didemnum vexillum requires hard substrata for successful colonization, therefore, it could colonize the bedrock and boulders that characterize this biotope. Didemnum vexillum can overgrow sessile organisms, including kelp Laminaria sp. However, no direct evidence was found on how Didemnum vexillum affects kelp or if it contributes to Laminaria sp. mortality (Järnegren et al., 2023), although epifaunal growth by Membranacea membrancea was reported to reduce the physical strength of kelp fronds (inc. Laminaria digitata) and make them susceptible to removal by wave action (Krumhansl et al., 2011). In addition, overgrowth by epiphytes contributed to the decline of Saccharina latissima in Norway (Andersen et al., 2011). However, Didemnum vexillum may compete for light and space with kelp and epifauna and could interfere with recruitment, which could lead to the mortality of some epifauna, the loss of kelp, and a reduction in biodiversity. Didemnum prefer sheltered conditions, so the wave-exposed and tidally swept conditions that characterize this biotope may mitigate its abundance. Therefore, a resistance of 'Medium' (some mortality, <25%) is suggested as a precaution. Resilience is likely to be 'Very low' as Didemnum vexillum would need to be physically removed to allow recovery. Hence, sensitivity to invasion by Didemnum is assessed as 'Medium'. However, confidence in the assessment is ‘Low’ due to the lack of direct evidence of damage to kelp beds.

The Pacific oyster, Magallana gigas

Evidence

The Pacific oyster, Magallana (syn. Crassostrea) gigas, is native to warm temperate regions from the northwest Pacific to Japan and northeast Asia, including Cape Mariya (Russia) to Hong Kong (China) (Carrasco & Baron, 2010; GBNNSIP, 2011, 2012). It is a fast-growing and tolerant species that has become a successful invader in the coastal waters of all continents, aside from Antarctica (Wrange et al., 2010; Carrasco & Baron, 2010; Padilla, 2010). 

It was initially introduced for aquaculture in Europe and the UK in the 1960s due to a decline in the Portuguese oyster (Crassostrea angulata) and the European flat oyster (Ostrea edulis) (Spencer et al., 1994; GBNNSIP, 2011, 2012; Humphreys et al., 2014, cited in Alves et al., 2021; Hansen et al., 2023). It was also introduced to the northeast Adriatic Sea (Ezgeta-Balic et al., 2019) and southwest England from France, possibly via fouling on ships (GBNNSIP, 2011, 2012; Padilla, 2010; Ezgeta-Balic et al., 2019).

Magallana gigas has a high fecundity, a long-lived pelagic larval phase (2 to 4 weeks) and can produce up to 200 million eggs during spawning (Herbert et al., 2012, 2016; Alves et al., 2021; Wood et al., 2021; Hansen et al., 2023). Hence, as a broadcast spawner, it has a high dispersal potential of more than 1000 km (Padilla, 2010; Wood et al., 2021). Although larval mortality can be as large as 99% due to sensitivity to environmental conditions (Alves et al., 2021), adults are long-lived so that populations can survive with infrequent recruitment (Padilla, 2010).

Larval dispersal has facilitated the establishment of populations in various regions, such as the Oosterschelde estuary in the Netherlands and the Scandinavian coastlines, where northward drift on tidal and wind-driven currents has been suggested (Hansen et al., 2023). Offshore structures and aquaculture operations can enhance spread (Wood et al., 2021).

Magallana gigas is an ecosystem engineer and can dramatically change habitat structure when it invades. Once successfully settled, groups of Pacific oysters may form dense aggregations, potentially forming a reef, which in some regions can reach densities of 700 individuals/m² (Herbert et al., 2012, 2016). Once, the density of live or dead Pacific oysters reaches or exceeds 200 ind./m², little of the underlying substratum remains visible (Herbert et al., 2016). These reefs can stabilize the sediment surface locally (Troost, 2010). When such reefs are formed or, particularly when the species colonizes soft sediments such as mud or sand, it can change and affect local communities, by creating hard substrata for mobile species, which might not otherwise be present before the invasion (Padilla, 2010). However, Hansen et al. (2023) suggested that no immediate ecosystem risk is observed where the Pacific oyster occurs sporadically.

Settlement requires hard substrata, including rock, bedrock, chalk, bare boulders, cobbles and pebbles and shells (Kochmann et al., 2012, 2013; McKinstry & Jensen, 2013; Herbert et al., 2016; Tillin et al., 2020). Magallana gigas also attaches to available hard materials in mixed sediment environments such as shingle and sand within otherwise unsuitable mudflats (Spencer et al., 1994; McKinstry & Jensen, 2013; Tillin et al., 2020).

Populations of Magallana gigas have been found on wave-exposed rocky shores to wave-sheltered soft sediment environments, and it has been described as a habitat generalist (Troost, 2010; Kochmann et al., 2012, 2013). For example, in Scotland, wild Magallana gigas are mainly located in the lower intertidal on bedrock, bedrock encrusted with barnacles, within bedrock crevices, and large and small boulders (Cook et al., 2014). Patches of Pacific oyster reefs have been recorded on littoral rock in Kent, southern England and on littoral sediments in southern England, the North Sea, and the English Channel (Herbert et al., 2012, 2016; Morgan et al., 2021).

Magallana gigas has been reported from estuaries growing on intertidal mudflats and sandflats, and other soft sediments (Padilla, 2010; Herbert et al., 2016; Cabral et al., 2020). The settlement of spat on hard substrata within sediments has been observed in the estuaries of the River Dart, Exe, Fal, Fowey, Tamar, Teign, and Yealm in Devon and Cornwall, the Menai Straits, Wales and large estuaries of Lough Swilly, Lough Foyle and the Shannon in Ireland, and the Tagus Estuary in Portugal (Spencer et al., 1994; Kochmann et al., 2012, 2013; Cabral et al., 2020). In Lough Swilly, Lough Foyle and the Shannon, the Pacific oyster was often associated with intertidal mud or sandflats (Kochmann et al., 2013). In contrast, the Pacific oysters were absent from sandflat areas in Poole Harbour (McKinstry & Jensens, 2013).

Although shorelines comprised mainly of mud were suggested to be unsuitable for spat settlement (Spencer et al., 1994), the presence of smaller hard substrata, such as shells or pebbles, can enable larvae to settle (Tillin et al., 2020). For example, in the River Teign estuary, Pacific oyster settlement was observed on shell-covered ground mainly attached to mussel shells, and occasionally attached to cockles, stones and common periwinkle (Littorina littorea) shells on a mud flat in the estuarine intertidal zone, otherwise mainly comprised of sand and mud (Spencer et al., 1994). In addition, the Blue Lagoon on the north shore of Poole Harbour had the highest abundance of oysters on mud mixed with shingle and shell (McKinstry & Jensen, 2013). Outside of the Blue Lagoon, oysters were also recorded on mixed substrata composed of mud, gravel, and shell (McKinstry & Jensen, 2013). Tillin et al. (2020) concluded that while successful invasions occurred on mudflats, Magallana gigas prefers mixed substrata. Fine mud sediments without hard substrata (such as small stones, gravel, and shell) are unlikely to be suitable (Tillin et al., 2020).

The speed of Magallana gigas reef formation on soft substrata seems to be dependent on the amount of hard substrata present (Troost, 2010). Bergstrom et al. (2021) reported that the presence of Magallana gigas was partially dependent on increasing gravel content up to 15% but remained stable with increasing percentages (measured up to 80%).

While often described as an intertidal and shallow subtidal species, Magallana gigas has been observed across a broader depth range. Although rocky habitats deeper than 10 m are generally considered unsuitable, it has been recorded down to 42 m in the Oosterschelde, Netherlands (Herbert et al., 2012, 2016; Tillin et al., 2020; Smaal et al., 2009).

It frequently occurs between Mean High Water and Mean Low Water in intertidal zones but has also been recorded at 1 to 10 m depth in regions like Sweden, Ireland, and the UK (Kochmann et al., 2013; Herbert et al., 2016; Bergstrom et al., 2021). In Lough Swilly and Lough Foyle, Ireland, oysters were found on shallow subtidal mussel beds and mixed mud and sand habitats (Kochmann, 2012). In the Thames Estuary and parts of Essex and Kent, oysters have also been found subtidally, 2–3 m below chart datum (Tillin et al., 2020).

Bergstrom et al. (2021) suggested the optimal depth in the Skagerrak is around 0.5 m, although presence is documented down to 5 m. In Lim Bay (Adriatic Sea), M. gigas occurs in the intertidal and shallow subtidal (down to 1 m), but not beyond 3 m depth (Stagličić et al., 2020). The species has not been recorded below extreme low water on rocky habitats, although it has been found subtidally on soft sediments in some areas (Herbert et al., 2012).

The Pacific oyster prefers wide intertidal areas with shallow gradients; it is generally absent from steep shores (McKinstry & Jensen, 2013; Herbert et al., 2016; Tillin et al., 2020). In Ireland and the Solway Firth, it is more commonly found on intertidal shores over 40–50 m wide (Kochmann et al., 2013; Cook et al., 2014).

It has been suggested that recruitment is enhanced, and abundances are higher in wave-sheltered conditions (Robinson et al., 2005 and Ruesink, 2007 cited in Teschke et al., 2020; Tillin et al., 2020). Teschke et al. (2020) found the abundance of Magallana gigas was significantly higher at wave-protected sites within the artificial harbours of Helgoland, North Sea, compared to wave-exposed sites outside the harbours. The authors suggested that the successful colonization in wave-protected sites could be due to the relative retention of water masses in the harbours that reduces larval drift and the whiplash effect on newly settled larvae. In addition, better growth and higher survival rates were observed at wave-protected sites, whereas mortality rates increased at wave-exposed sites, due to the wave exposure causing dislodgement or detachment from the settlement substratum (Teschke et al., 2020; Tillin et al., 2020). Similarly, Bergstrom et al. (2021) noted that the occurrence of high densities of both Ostrea edulis and Magallana gigas decreased with increasing wave exposure.

Magallana gigas can withstand a wide range of salinities (from 11 to 34 PSU), but no oysters were observed in areas on the west Swedish coast which had salinities less than 20 PSU (Wrange et al., 2010; Kochmann, 2012; Chu et al., 1996 cited in Tillin et al., 2020). Bergstrom et al. (2021) noted that in the Skagerrak, native and Pacific oyster densities increased with rising salinity above 15 to 27 PSU. Larvae can survive salinities between 19 and 35 PSU (Troost, 2010; Tillin et al., 2020). Growth of Pacific oysters can occur between 10 and 30 PSU (Troost, 2010).

Carrasco & Baron (2010) suggested that Magallana gigas has successfully adapted to colonize a range of thermal niches. Temperature is important for the life cycle of the Pacific oyster and influences the establishment of feral and wild populations (Alves et al., 2021). Within its native range, Magallana gigas occurs in areas where the sea surface temperatures range from 14.0°C to 28.6°C in the warmest month of the year, and between -1.9°C and 19.8°C in the coldest month (Carrasco & Baron, 2010).

Magallana gigas has a seasonal reproductive cycle (Alves et al., 2021). Spawning occurs in the summer months, when temperatures are 16 to 34°C and larvae require a water temperature of 18°C or above for successful development (Mann 1979; Troost, 2010; Kochmann, 2012; Ezgeta-Balic et al., 2020; Alves & Tidbury, 2022). In Poole, UK, spawning temperatures were estimated at 19.7°C (Alves & Tidbury, 2022). Ezgeta-Balic et al.‘s (2020) study indicated that temperatures in the Mediterranean and the Adriatic were favourable for Pacific oyster larval development, with gametogenesis initiated at temperatures from around 10 to 15°C and spawning initiated at around 24°C. However, the lower thermal limit for spawning was recognized as 16°C (Carrasco & Baron, 2010) and once settled, larvae are unable to survive in temperatures below 3°C (Alves & Tidbury, 2022).

Adults can survive in water temperatures up to 40°C and at low tide, freezing air temperatures as low as -17°C, depending on the salinity of the water in their shells (Troost, 2010; Tillin et al., 2020; Hansen et al., 2023). Growth of Pacific oysters occurs between 3 and 40°C (Troost, 2010; Kochmann, 2012).

Dense macroalgal cover is unsuitable for the Magallana gigas (Herbert et al., 2012, 2016; Tillin et al., 2020), being rarely found under macroalgal cover in Northern Ireland, absent from exposed bedrock or large boulders with macroalgae cover in the Solway Firth, Scotland, and absent in Poole Harbour, where there was competition with macroalgae (Kochmann et al., 2012, 2013; McKinstry & Jensen, 2013; Cook et al., 2014; Tillin et al., 2020). Fucus cover significantly reduced larval recruitment of the Pacific oyster in the Wadden Sea (Diederich, 2005). Hence, the Pacific oyster is more likely to colonize bare rock, boulders, or mussel beds without macroalgae (Diederich, 2005; Cook et al., 2014). Kochmann et al. (2013) suggested that macrophyte canopies prevent larvae from settling on the rock underneath, and macroalgae fronds inhibit settlement and recruitment by exuding metabolites.

Sensitivity assessment. While most of the evidence suggests the environmental conditions within this biotope are suitable for Magallana gigas, it is unlikely that they would be able to colonize this biotope without the removal of the kelp canopy. In addition, populations may be limited to low densities due to very wave exposed to wave exposed conditions. Therefore, this biotope is assessed as ‘Not Sensitive’ to this pressure.

Wireweed, Sargassum muticum

Evidence

Competition with invasive macroalgae may be a potential threat to this biotope (de Bettignies et al., 2021). Sargassum muticum is a circumglobal invasive species (Engelen et al., 2015). It is recorded (2015) from Norway to Morocco and into the Mediterranean in the eastern Atlantic and from Alaska to Baja California in the eastern Pacific and from southern Russia to southern China in the western Pacific (Engelen et al., 2015). It colonizes a variety of habitats and can tolerate -1°C to 30°C and survive salinities below 10 ppt. Although fertilization does not occur below 15 ppt and growth of germlings is limited below 10°C, it can complete its life cycle as long as temperatures are over 8°C for at least four months of the year (Engelen et al., 2015). However, its distribution is limited by the availability of hard substratum (e.g. stones >10 cm) and light (Staeher et al., 2000; Strong & Dring, 2011; Engelen et al., 2015). It is most abundant between 1 and 3 m below mean water, but it has been recorded at 18 m or 30 m in the clear waters of California. However, it is a poor competitor under low light and only develops dense canopies in shallow areas (Engelen et al., 2015).

Sargassum muticum was shown to replace and out-compete leathery, canopy-forming macroalgae such as Saccharina latissima, Halidrys siliquosa, and Fucus spp. and, to a lesser degree, understorey species such as Codium fragile, Chondrus crispus and Dictyota dichotoma in Limfjorden, Denmark, between 1984 and 1997 (Staehr et al., 2000; Engelen et al., 2015; de Bettignies et al., 2021). The invasion in Limfjorden had stabilized by 2005, although many of the native macroalgal species continued to decline (Engelen et al., 2015). In Limfjorden, the distribution of Sargassum muticum was limited to areas with hard substratum, in particular stones > 10 cm in diameter, while smaller stones, gravel and sand were unsuitable. It was most abundant between 1 and 4 m in depth but had low cover at 0-0.5 m or 4-6 m, in the turbid waters of the Limfjorden.  Limfjorden is wave sheltered, although wave exposure has been reported to restrict the growth and survival of Sargassum muticum (Staehr et al., 2000). Viejo et al. (1995) reported that Sargassum muticum transplanted to wave-exposed shores in Spain experienced >80% breakages within a month and that the growth of undamaged plants was significantly lower than that of plants on sheltered shores. Similarly, Andrew & Viejo (1998) noted that Sargassum muticum was restricted to intertidal rockpools in wave-exposed sites in the Bay of Biscay.

Strong & Dring (2011) used canopy removal experiments to investigate inter- and intra-species competition between Sargassum muticum and Saccharina latissima in the Dorn, Strangford Lough, N. Ireland. The Dorn consists of tidal pools, very sheltered from wave action but with moderately strong tidal streams (1-2 knots). Sargassum muticum grew better in mixed stands with Saccharina latissima than in the highest-density monospecific stands examined. However, the growth of Saccharina was not affected by the proportion of Sargassum in mixed stands. They concluded that Saccharina was not impacted significantly by the alien species, while Sargassum benefited from growth in mixed stands. Experimental manipulation of subtidal algal canopies in San Juan Islands, Washington State, USA, showed that Sargassum muticum reduced the abundance of native macroalgae, including the kelp Laminaria bongardiana, due to shading. However, experimental removal of Sargassum resulted in the recovery of native species within about one year (Britton-Simmons, 2004; Engelen et al., 2015). The negative effects of Sargassum muticum on native macroalgae are mainly due to competition for light, rather than changes in nutrient availability, sedimentation or water flow (Britton-Simmons, 2004; Engelen et al., 2015).  

Sensitivity assessment. The above evidence suggests that Sargassum muticum can both compete with and co-exist with Saccharina latissima, depending on local conditions. For example, Sargassum muticum out-competed Saccharina latissima in the Limfjorden but coexisted in the Dorn in Strangford Lough.

This Saccharina latissima dominated biotope (SS.SMp.KSwSS.SlatR) is found at variable depth (0-30 m JNCC, 2015) and full salinity with moderately strong to weak tidal streams and extreme wave exposure to sheltered conditions. The evidence above suggests that Sargassum also prefers wave-sheltered conditions and shallow water (ca 1 to 4 m depth). Therefore, Sargassum muticum is only likely to threaten the shallowest (e.g. 0-5 m) and wave-sheltered examples of this biotope, where suitable hard substrata are available. Sargassum muticum may either co-exist with or out-compete Saccharina latissima, resulting in a potentially significant (25-75%) reduction in the abundance or extent of the native kelp and a possible decrease in the diversity of other macroalgae. Therefore, resistance is assessed as ‘Low’ for shallow, wave-sheltered examples of the biotope, i.e. above 5 m in depth, while resistance is probably ‘High’ for examples below 5 m. Recovery after invasion by Sargassum, although rapid, would require direct intervention (removal) so that resilience is assessed as ‘Very low’. Hence, the sensitivity of shallow, sheltered examples of the biotope is assessed as ‘High’. Overall, confidence is assessed as ‘Low’ due to evidence of variation and the site-specific nature of competition between native kelps and Sargassum muticum.

Wakame, Undaria pinnatifida

Evidence

Competition with invasive macroalgae may be a potential threat to this biotope (de Bettignies et al., 2021). Undaria pinnatifida (Wakame or Asian kelp) is a large brown seaweed and an Invasive Non-Indigenous Species (INIS) that could out-compete native UK kelp species (see Farrell & Fletcher, 2006; Thompson & Schiel, 2012; Brodie et al., 2014; Hieser et al., 2014; Arnold et al., 2016; Epstein & Smale, 2017; Epstein & Smale, 2018; Kraan, 2017; Epstein et al., 2019a,b; Tidbury, 2020). Undaria pinnatifida originates from Japan but is currently established on the coastlines of New Zealand, Australia, Northern France, Spain, Italy, the UK, Portugal, Belgium, Holland, Argentina, Mexico, and the USA (De Leij et al., 2017). Undaria pinnatifida was first recorded in the UK in the Hamble Estuary in 1994 (Macleod et al., 2016). It has since proliferated along UK coastlines.  One year after its discovery at the Queen Anne Battery marina, Plymouth, it had become a major fouling plant on pontoons (Minchin & Nunn, 2014). Although initially restricted to artificial habitats, such as marinas and ports, it is now widespread in natural habitats in several areas, including Plymouth Sound.

Undaria pinnatifida seems to settle better on artificial substrata (e.g. floats, marinas, or piers) than on natural rocky shores among local kelps (Vaz-Pinto et al., 2014). It is found predominantly in low intertidal to shallow subtidal habitats (Epstein et al., 2019b) and is significantly more abundant on artificial substrata compared to natural rocky substrata (Heiser et al., 2014; Epstein & Smale, 2018).  James (2017) suggested that Undaria pinnatifida could out-compete native species on artificial substrata (such as marinas and wharf structures). In Plymouth, UK, De Leij et al. (2017) found that natural habitats with dense native macroalgal canopies, such as Laminaria hyperborea, Laminaria ochroleuca, Laminaria digitata, and Saccharina latissima, had more resistance to Undaria pinnatifida invasion than disturbed or sparse canopies, due to limited space and light availability for Undaria pinnatifida recruits. However, the dense canopies did not always prevent the invasion of Undaria pinnatifida as sporophytes were still recorded within dense Laminaria canopies, so canopy disturbance was not always required (De Leij et al., 2017; Epstein & Smale, 2018).

Undaria pinnatifida species behaves as a winter annual, and recruitment occurs in winter, followed by rapid growth through spring, maturity, and then senescence through summer, with only the microscopic life stages persisting through autumn. It exhibits multiple dispersal strategies, such as short-range spore dispersal and long-range dispersal as whole drift plants or fragments. Undaria pinnatifida has spread rapidly across the UK and Europe, resulting in community-wide responses and impacts (Vaz-Pinto et al., 2014; Epstein & Smale, 2017). Its impacts are complex and context-specific, depending on space, time, and taxa present in the introduced location (Epstein & Smale, 2017; Teagle et al., 2017; Tidbury, 2020).

Undaria pinnatifida has a wide physiological niche, meaning it can occur in both coastal and estuarine environments, showing tolerance for varying salinities, turbidity, and siltation (Heiser et al., 2014; Epstein & Smale, 2018). Undaria pinnatifida can inhabit a broad range of habitats, including reefs; coastal brackish/saline lagoons; large shallow inlets and bays; estuaries; estuarine rocky habitats; natural or near-natural estuary; coastal lagoons; and tidal rivers, estuaries, mudflats, sandflats and lagoons (James 2017). Undaria pinnatifida prefers sites sheltered with low wave exposure and weak tidal streams (Heiser et al., 2014; Epstein & Smale, 2018). In natural habitats, Undaria pinnatifida was not recorded if the wave fetch was greater than 642 km, but increased in abundance and cover in very sheltered sites (Epstein & Smale, 2018).

In Plymouth Sound (UK), Epstein et al. (2019b) found that within its depth range (+1 to –4 m), Undaria pinnatifida co-existed with seven species of canopy-forming brown macroalgae, including Saccharina latissima. However, they reported that Undaria pinnatifida biomass was negatively related to Saccharina latissima in both intertidal and subtidal habitats. This was only statistically significant in subtidal habitats, which suggested that there was some competition between the two species (Epstein et al., 2019b). Heiser et al. (2014) surveyed 17 sites within Plymouth Sound, UK and found that Saccharina latissima was significantly more abundant at sites with Undaria pinnatifida, with ca 5 Saccharina latissima individuals present per m², compared to ca 0.5 Saccharina latissima individuals per m² present at sites without Undaria pinnatifida.

Undaria pinnatifida has been reported to both co-exist with and out-compete Saccharina latissima (Farrell & Fletcher, 2006; Heiser et al., 2014; Epstein et al., 2019b). For example, in Torquay Marina, UK, Farrell & Fletcher (2006) completed a canopy removal experiment between 1996 and 2002. They reported that Saccharina latissima decreased in both control and treatment plots from ca 3 plants per 0.45 m² in 1996 to ca 1 plant per 0.45 m² in 1997 and had disappeared completely from pontoons by 2002. This coincided with a significant increase in Undaria pinnatifida from zero plants per 0.45 m² in 1996 to ca 6 plants per 0.45 m² in 1997. However, there was a slight decrease in Undaria pinnatifida in both control and treatment plots between 1997 and 1998. By 2002, Undaria pinnatifida had recovered at control and treatment plots to ca 4-6 plants per 0.45 m², whereas Saccharina latissima had not.

Undaria pinnatifida was successfully eradicated on a sunken ship in Clatham Islands, New Zealand, by applying a heat treatment of 70°C (Wotton et al., 2004). However, numerous other eradication attempts have failed and as noted by Fletcher & Farrell (1998), once established, Undaria pinnatifida resists most attempts at long-term removal.

The proliferation of Undaria pinnatifida and competition with native species may cause a reduction in local biodiversity (Valentine & Johnson, 2003; Vaz-Pinto et al., 2014; Arnold et al., 2016; Teagle, 2017; Tidbury, 2020). A shift towards Undaria pinnatifida-dominated beds could result in diminished epibiotic assemblages and lower local biodiversity compared with assemblages associated with native perennial kelp species, such as Laminaria spp. and Saccharina latissima (Arnold et al., 2016; Teagle et al., 2017). In Plymouth, UK, Arnold et al. (2016) found that Undaria pinnatifida supported less than half the number of taxa and had no unique epibionts compared to Laminaria ochroleuca and Saccharina latissima (Arnold et al., 2016).

Sensitivity assessment. The above evidence suggests that Undaria pinnatifida can both compete with and co-exist with Saccharina latissima, depending on local conditions. For example, Undaria pinnatifida can out-compete competitive invasive species like Saccharina latissima in artificial habitats, such as in Torquay Marina, but within natural habitats, it can co-exist with native kelp species within its depth range (-1 to 4 m), as shown in Plymouth Sound, UK. 

This Saccharina latissima dominated biotope (SS.SMp.KSwSS.SlatR) is found at variable depth (0-30 m JNCC, 2015) and full to variable salinity with moderately strong to weak tidal streams and extreme wave exposure to sheltered conditions. The evidence above suggests that Undaria prefers sheltered conditions, with a low tidal flow, in the shallow subtidal and sublittoral fringe (ca +1 to 4 m in depth). Therefore, Undaria pinnatifida is only likely to threaten the shallowest (e.g. 0-5 m) and wave-sheltered examples of this biotope, where suitable hard substrata are available. Undaria pinnatifida may either co-exist with or out-compete Saccharina latissima, resulting in a potentially significant (25-75%) reduction in the abundance or extent of the native kelp and a possible decrease in the diversity of other macroalgae. Therefore, resistance is assessed as ‘Low’ for shallow, wave sheltered examples of the biotope, i.e. above 5 m in depth, while resistance is probably ‘High’ in examples below 5 m. Recovery after invasion by Undaria, although rapid, would require direct intervention (removal) so that resilience is assessed as ‘Very low’. Hence, the sensitivity of shallow, sheltered examples of the biotope is assessed as ‘High’. Overall, confidence is assessed as ‘Low’ due to evidence of variation and the site-specific nature of competition between native kelps and Undaria pinnatifida.

Other INIS

Evidence

The golden kelp Laminaria ochroleuca is a warm-temperate Lusitanian kelp with a distribution ranging from Morocco to the south of the UK. It was first recorded in the southwest UK in 1946 (Parke, 1948) and is projected to expand further northwards under future climate change scenarios (Franco et al., 2018). A small population was recorded in northwest Ireland in 2018 (Schoenrock et al., 2019), further suggesting ongoing poleward expansion. In Plymouth Sound, southwest UK, estimates of Laminaria ochroleuca standing stock are now comparable to those of the native kelp Laminaria hyperborea (Taylor-Robinson et al., 2024; also see Smale et al., 2016 for the standing stock of Laminaria hyperborea). While not considered a traditional invasive species, its northward expansion into the UK has led to competition with native kelps such as Laminaria hyperborea, and since the environmental range of Saccharina latissima overlaps with Laminaria hyperborea, Laminaria ochroleuca can be considered as a potential invasive competitor. No evidence could be found on the interactions of Laminaria ochroleuca on Saccharina latissima, so evidence for Laminaria hyperborea is presented instead.

It is suggested that Laminaria ochroleuca may have a competitive advantage over Laminaria hyperborea due to its tolerance of warmer waters. Barrientos et al. (2025) investigated changes in kelp forests in northwest Spain between 1997 and 2023. They found that kelp forests had disappeared or severely declined in density at 29 of 50 sites, and the canopy was now dominated by Laminaria ochroleuca at the surviving sites, while Laminaria hyperborea is almost entirely absent, occurring at only two sites. These changes were linked to sea surface temperature (an average increase of 0.01 to 0.02°C per year over the 26-year study period), which suggested that Laminaria ochroleuca was more resistant to warming and could, therefore, outcompete Laminaria hyperborea under global warming scenarios.

Saccharina latissima grows well between 5 and 17°C (Druehl, 1967, Fortes & Luning, 1980 and Machalek, Davison & Falkowski, 1996 cited in Kerrison et al., 2015) and has a limiting temperature isotherm of 19 to 20°C (Müller et al., 2009). However, temperature ecotypes exist, having adapted to high seasonal temperature exposure: populations from Helgoland, Germany, can tolerate temperatures of 18 to 20°C (Davison, 1987 cited in Kerrison et al., 2015), while populations in New York, USA, can survive at >20°C, albeit with substantially reduced growth (Gerard & Du Bois, 1988). In addition, Azevedo et al. (2016) cultured Saccharina latissima in tanks in northwest Portugal throughout the summer, withstanding average temperatures around 20°C from May onwards (temperature varied between 11.7°C in April and 24.9°C in August), well above published optimum temperatures for this species (10 to 15°C). Therefore, Saccharina latissima may be able to compete alongside Laminaria ochroleuca better than Laminaria hyperborea, but both native species share a similar geographic range from Portugal to the Arctic (Birkett et al., 1998b; Azevedo et al., 2016).

There is contrasting evidence on the relative resilience of Laminaria ochroleuca, Laminaria hyperborea, and Saccharina latissima to storm damage. Pereira et al. (2017) reported no recovery of Laminaria hyperborea populations in the two years following a storm in northern Portugal, whereas Laminaria ochroleuca showed partial recovery. In contrast, during the North East Atlantic storm season of 2013 to 2014, the south coast of the UK was subjected to some of the most intense storms in recent history, being classed as a '1-in-30 year' event, where inshore significant wave heights and periods exceeded 7 m and 13 seconds (Smale & Vance, 2015). Overall, kelp canopies were highly resistant to storm disturbance, however, at one study site, a mixed canopy comprising Laminaria ochroleuca, Saccharina latissima, and Laminaria hyperborea was significantly altered by the storms, due to a decreased abundance of the former two species (Smale & Vance, 2015). In particular, the breakage of mature Laminaria hyperborea stipes ranged between 2.3 and 6.9%, while broken Laminaria ochroleuca stipes were on average 8.7 times more prevalent (Smale & Vance, 2015). Given this conflicting evidence, it remains unclear whether Laminaria ochroleuca biotopes could displace Laminaria hyperborea and Saccharina latissima biotopes following storm events.

Another potential advantage of Laminaria ochroleuca is its greater average stipe length compared to Laminaria hyperborea, potentially reducing light availability for Laminaria hyperborea recruits in mixed-population forests (Smale et al., 2014). This shading effect may exaggerate the impacts of marine heatwaves on Laminaria hyperborea, as elevated temperatures increase metabolic demands that cannot be met under light-limited conditions (Bass et al., 2023). However, in terms of Saccharina latissima, Laminaria ochroleuca, on average, is shorter (1.5 m compared to 4 m – see Laminaria ochroleuca and Saccharina latissima), and may be at a lower risk of shading.

The introduction of Laminaria ochroleuca into Laminaria hyperborea or Saccharina latissima forests can have negative impacts on biodiversity. Kelp stipe assemblages differ significantly between Laminaria ochroleuca and Laminaria hyperborea due to the texture of the stipe. Laminaria hyperborea stipes are rough and pitted and, therefore, have a larger surface area, while Laminaria ochroleuca stipes are uniformly smooth. Teagle & Smale (2018) found species from up to 15 different taxonomic groups on Laminaria hyperborea stipes in spring, compared to 2 taxa at most on Laminaria ochroleuca stipes all year round. In addition, the biomass of Laminaria hyperborea stipe assemblages was >3,600 more than that of Laminaria ochroleuca stipe assemblages. Therefore, the proliferation of Laminaria ochroleuca could reduce available habitat space for epibionts that are associated with Laminaria hyperborea biotopes, and potentially Saccharina latissima biotopes as well, as 111 taxa have previously been recorded on Arctic Saccharina latissima individuals (Shunatova et al., 2018 cited in Diehl et al., 2024).

Sensitivity Assessment. Considering Laminaria hyperborea in the place of Saccharina latissima, and the evidence for the poleward range shift for Laminaria hyperborea and Saccharina latissima (Moy & Christie, 2012; Assis et al., 2016; Casado-Amezúa et al., 2019; Simkanin et al., 2005 cited in Veenhof et al., 2024), alongside the expansion of Laminaria ochroleuca into higher latitudes (Franco et al., 2018), Laminaria ochroleuca could displace existing kelp biotopes in the southern UK. In Plymouth Sound, Laminaria ochroleuca is already rivalling Laminaria hyperborea, which used to be the dominant kelp in the area (Saccharina latissima is also common in this region) (Smale et al., 2014; Taylor-Robinson et al., 2024). Its stipe length could reduce light availability for smaller and/or juvenile kelps, and when combined with elevated temperatures, could create unfavourable conditions for the persistence and recovery of native species. Laminaria ochroleuca, however, does form mixed forests with Laminaria hyperborea and Saccharina latissima in moderately sheltered to exposed shores, and has physiological and morphological advantages that could allow it to proliferate if Laminaria hyperborea and/or Saccharina latissima density was reduced. Resistance to Laminaria ochroleuca is assessed as ‘Low’ based on the evidence of Laminaria ochroleuca rivalling Laminaria hyperborea in Plymouth Sound, southwest UK, and potentially rivalling Saccharina latissima. Hence, resilience is assessed as ‘Very Low’, and sensitivity as ‘High’. While the quality and applicability of the evidence is high, there is contrasting evidence regarding both species’ resistance and resilience to storm damage. Therefore, confidence in this sensitivity assessment is ‘Medium’.